The Essentials for Using COM in Managed Code

 This Chapter
  • Referencing a COM Component in Visual Studio .NET

  • Referencing a COM Component Using Only the .NET Framework SDK

  • Example: A Spoken Hello, World Using the Microsoft Speech API

  • The Type Library Importer

  • Using COM Objects in ASP.NET Pages

  • An Introduction to Interop Marshaling

  • Common Interactions with COM Objects

  • Using ActiveX Controls in .NET Applications

  • Deploying a .NET Application That Uses COM

  • Example: Using Microsoft Word to Check Spelling

So far we've covered the features of the .NET Framework, seen some managed code, and discussed how interoperating with COM works from a high level. This chapter shows what COM Interoperability means when sitting down at the keyboard and writing managed code that uses COM components. By the end of this chapter, you'll have all the knowledge you need to use many COM components in your .NET applications.

Before going into all the details on using COM components in .NET, let's see how Visual Studio .NET makes referencing a COM component fairly seamless. As mentioned in the previous chapter, one of the great benefits of COM access in the .NET Framework is that tons of COM components already exist that can fill in missing functionality in the pure .NET world. So, the first task in this chapter is to change the Hello, World programs of Chapter 1, "Introduction to the .NET Framework," into audible Hello, World programs using the Microsoft Speech API (SAPI) COM component. Of course, this requires a computer with speakers and the appropriate hardware. Also, if you don't already have it, download the Microsoft Speech Software Development Kit (SDK) version 5.1 from Microsoft's Web site and follow its instructions to install the software. Windows XP has the necessary software pre-installed.

Caution -

Using COM components authored in Visual Basic 6 may crash during shutdown when used from a .NET application unless you have at least Service Pack 3 for the Visual Basic 6 runtime. If you have Visual Studio 6, you can download and install a Visual Studio 6 Service Pack. Otherwise, you can download Service Pack 5 for just the VB6 runtime at EN-US/VBRun60sp5.exe. Windows 2000 and later operating systems already have a runtime with the necessary update.

Referencing a COM Component in Visual Studio .NET

Referencing a COM component is often the first step for using it in Visual Studio .NET, and it leads to the audible Hello, World sample later in this chapter. When referencing a COM component, you're actually referencing its type library, which provides the compiler with definitions of types, methods, and properties that the component exposes.

Digging Deeper -

Type libraries can be standalone files with a .tlb extension, or they can be embedded in a file as a resource. They are typically contained in files with the extension .dll, .exe, .ocx, or .olb. A quick way to check for the presence of a type library in a file is to try opening the file in OLEVIEW.EXE, the utility introduced in the previous chapter.

Type libraries are often, but not always, registered in the Windows Registry. Among other things, this enables programs such as Visual Studio .NET to present a user with a list of type libraries on the current computer.

The steps for referencing a COM component's type library are:

  1. Start Visual Studio .NET and create either a new Visual Basic .NET console application or a Visual C# console application. The remaining steps apply to either language.

  2. Select the Project, Add Reference... menu. There are two kinds of components that can be referenced: .NET and COM. The default .NET tab shows a list of assemblies. To add a reference to a COM component, click the COM tab (see Figure 3.1). This tab should be familiar to Visual Basic 6 users, who reference a COM component's type library the same way (using the Project, References... menu in the Visual Basic 6 IDE).

  3. The list of components consists of type libraries that are registered in the Windows Registry (under HKEY_CLASSES_ROOT\TypeLib). Each name displayed is the contents of the library's helpstring attribute, or the library name if none exists. To add a reference to a type library that isn't listed, click Browse... and select the file. This registers the type library, so the next time it is listed in the dialog. You should double-click Microsoft Speech Object Library in the list to try the upcoming example.

  4. 3. Click the OK button.

Figure 3.1 Adding a reference to a COM component using Visual Studio .NET, and the same task in Visual Basic 6.

Digging Deeper -

It may surprise you to learn that the list of assemblies on the Add Reference dialog's .NET tab is obtained from the Windows Registry, not from the Global Assembly Cache. The GAC is not meant to be used at development time, so Visual Studio .NET uses the registry to find places in the file system to search for assemblies.

To add your own assemblies to the list, you can add a new subkey under the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\AssemblyFolders key (or the same key under HKEY_CURRENT_USER) with any name, such as "My Company". Its default value should be set to the name of a directory, such as "C:\Program Files\My Company\". Any assemblies (with a .dll extension) that reside in this directory are loaded and listed with its assembly name, or the string inside its System.Reflection. AssemblyTitleAttribute custom attribute if it exists.

If adding the reference succeeded, a SpeechLib reference appears in the References section of the Solution Explorer window, shown in Figure 3.2.

Figure 3.2 The Solution Explorer after a reference has been added.

You can use the Object Browser (by clicking View, Other Windows, Object Browser) to view the contents of the SpeechLib library, as shown in Figure 3.3.

Figure 3.3 Browsing the type information for a COM component.

Referencing a COM Component Using Only the .NET Framework SDK

Now, let's look at how to accomplish the same task using only the .NET Framework SDK. The following example uses the .NET Framework Type Library to Assembly Converter (TLBIMP.EXE). This utility is usually just called the type library importer, which is what TLBIMP stands for. As mentioned in Chapter 1, if you've installed Visual Studio .NET but still want to run the SDK tools, you may have to open a Visual Studio .NET command prompt to get the tools in your path.

The steps for referencing a COM component without Visual Studio .NET are:

  1. From a command prompt, run TLBIMP.EXE on the file containing the desired type library, as follows (replacing the path with the location of SAPI.DLL on your computer):

    TlbImp "C:\Program Files\Common Files\Microsoft Shared\Speech\sapi.dll"

    This example command produces an assembly in the current directory called SpeechLib.dll because SpeechLib is the name of the input library (which can be seen by opening SAPI.DLL using the OLEVIEW.EXE utility). A lot of warnings are produced when running TLBIMP.EXE, but you can ignore them.

  2. Reference the assembly just as you would any other assembly, which depends on the language. Using the command-line compilers that come with the SDK, referencing an assembly is done as follows:

    C# (from a command prompt):

      csc HelloWorld.cs /r:SpeechLib.dll 

    Visual Basic .NET (from a command prompt):

      vbc HelloWorld.vb /r:SpeechLib.dll 

    Visual C++ .NET (in source code):

      #using <SpeechLib.dll> 

    After TLBIMP.EXE has generated the assembly, you can browse its metadata using the IL Disassembler (ILDASM.EXE) by typing the following:

    ildasm SpeechLib.dll

The name IL Disassembler is a little misleading because it's really useful for browsing metadata, which is separate from the MSIL. As shown in Figure 3.4, most SpeechLib methods don't even contain MSIL because the methods are empty implementations that forward calls to the original COM component. This can be seen by double-clicking on member names.

Figure 3.4 Using the IL Disassembler as an object browser.

ILDASM.EXE gives you much more information than the object browser in Visual Studio .NET, including pseudo-custom attributes. The information in the windows that appear when you double-click on items is explained in detail in Chapter 7, "Modifying Interop Assemblies."

Example: A Spoken Hello, World Using the Microsoft Speech API

Once you've either referenced the Microsoft Speech Object Library in Visual Studio .NET, or run the TLBIMP.EXE utility on SAPI.DLL, you're ready to write the code that uses this COM component. This can be done with these two steps:

  1. Type the following code either in a Visual Studio .NET project or in your favorite text editor.

    The C# version (HelloWorld.cs):

    using SpeechLib;
    class Class1
     static void Main()
     SpVoice voice = new SpVoice();
     voice.Speak("Hello, World!", SpeechVoiceSpeakFlags.SVSFDefault);

    The Visual Basic .NET version (HelloWorld.vb):

    Imports SpeechLib
    Module Module1
     Sub Main()
     Dim voice as SpVoice
     voice = new SpVoice()
     voice.Speak("Hello, World!")
     End SubEnd Module

    The C++ version (HelloWorld.cpp):

    #using <mscorlib.dll> // Required for all managed programs
    #using <SpeechLib.dll> // The assembly created by TLBIMP.EXE
    using namespace SpeechLib;
    void main()
     SpVoice* voice = new SpVoiceClass();
     voice->Speak("Hello, World!", SpeechVoiceSpeakFlags::SVSFDefault);};

    Because Visual C++ .NET projects do not provide a mechanism for referencing COM components, TLBIMP.EXE needs to be used to create SpeechLib.dll regardless of whether or not you use Visual Studio .NET.

  2. Compile and run the code (and listen to the voice). Feel free to have some more fun with the Speech API. You'll find that interacting with it is easy after you've gotten this far.

Notice the differences between the C#, Visual Basic .NET, and C++ versions of the same program. For example, the C# and C++ calls to Speak use two parameters but the same call in VB .NET has just one parameter. That's because the second parameter is optional, yet C# and C++ do not support optional parameters. Instead, a value must always be passed. Also notice that the C++ program instantiates SpVoiceClass instead of SpVoice as the others do. In this case, the C# and VB .NET compilers are doing some extra work behind-the-scenes to enable the user to work with a class that has the same name as the original COM coclass. More information about this extra work is given in the next chapter, "An In-Depth Look at Imported Assemblies."

The Type Library Importer

COM components such as the Microsoft Speech API expose type information using type libraries, but .NET compilers (excluding Visual C++ .NET) don't understand type libraries. They only understand .NET metadata as a source of type information. Therefore, the key to making a COM component readily available to the .NET world is a mechanism that takes a type library and produces equivalent metadata. The Common Language Runtime's execution engine contains this functionality, which is called the type library importer.

The term type library importer was introduced previously when describing TLBIMP.EXE, but the type library importer is exposed in other ways as well. In fact, when you add a reference to a COM component in Visual Studio .NET, you are really invoking the type library importer. This creates a new assembly, and this assembly—not the type library—is what your project is really referencing. This assembly (SpeechLib.dll in the previous example) is the same file that would have been generated by TLBIMP.EXE. It's always a single file with a .dll extension, and is placed in a directory specific to your project when generated by Visual Studio .NET.

Interop Assemblies

An assembly produced by the type library importer is known as an Interop Assembly because it contains definitions of COM types that can be used from managed code via COM Interoperability. The metadata inside an Interop Assembly enables .NET compilers to resolve calls, and enables the Common Language Runtime to generate a Runtime-Callable Wrapper (RCW) at run time.

You can think of importing a type library as providing a second view of the same COM component—one for unmanaged clients and another for managed clients. This relationship is shown in Figure 3.5.

Figure 3.5 Two views of the same COM component.

The actual implementation of the COM component remains in the original COM binary file(s). In other words, nothing magical happens to transform unmanaged code into the MSIL produced by a .NET compiler. Unlike normal assemblies, which contain an abundance of both metadata and MSIL, Interop Assemblies consist mostly of metadata. The signatures have special custom attributes and pseudo-custom attributes that instruct the CLR to delegate calls to the original COM component at run time.

There are three ways of using the type library importer to create an Interop Assembly:

  • Referencing a type library in Visual Studio .NET.

  • Using TLBIMP.EXE, the command-line utility that is part of the .NET Framework SDK.

  • Using the TypeLibConverter class in the System.Runtime.InteropServices namespace.

All three of these methods produce the exact same output, although each option gives more flexibility than the previous one. TLBIMP.EXE has several options to customize the import process, but Visual Studio .NET doesn't expose these customizations when referencing a type library. Using TLBIMP.EXE to precisely mimic the behavior of Visual Studio .NET when it imports a type library, you'd need to run it as follows:

TlbImp InputFile /out:Interop.LibraryName.dll /namespace:LibraryName /sysarray

where InputFile is the file containing the type library (such as SAPI.DLL) and LibraryName is the name found inside the type library that can be viewed with OLEVIEW.EXE (such as SpeechLib). All of TLBIMP.EXE's options are covered in Appendix B, "SDK Tools Reference."

The TypeLibConverter class enables programmatic access to type library importing, and has one additional capability when compared to TLBIMP.EXE—importing an in-memory type library. The use of this class is demonstrated in Chapter 22, "Using APIs Instead of SDK Tools."

Some of the transformations made by the type library importer are non-intuitive. For example, every COM class (such as SpVoice) is converted to an interface, then an additional class with the name ClassNameClass (such as SpVoiceClass) is generated. More information about this transformation is given in the next chapter, "An In-Depth Look at Imported Assemblies," but this is why SpVoiceClass had to be used in the C++ Hello, World example. The C# and VB .NET compilers perform a little magic to enable instantiating one of these special interfaces such as SpVoice, and treats it as if you're instantiating SpVoiceClass instead.

Whereas the process of converting type library information to metadata is called importing, and the process of converting metadata to type library information is called exporting. Type library export is introduced in Chapter 8, "The Essentials for Using .NET Components from COM."

Although TLBIMP.EXE has several options that Visual Studio .NET doesn't allow you to configure within the IDE, you can reference an Interop Assembly in two steps in order to gain the desired customizations within Visual Studio .NET. First, produce the Interop Assembly exactly as you wish using TLBIMP.EXE. Then, reference this assembly within Visual Studio .NET using the .NET tab instead of the COM tab. Simply browse to the assembly and add it to your project, just as you would add any other assembly.

Everything so far assumes that a type library is available for the COM component you want to use. For several existing COM components, this is simply not the case. If the COM component has one or more IDL files, you could create a type library from them using the MIDL compiler. (Unfortunately, there's no such tool as IDLIMP that directly converts from IDL to .NET metadata.) As mentioned previously in Figure 3.5, however, .NET compilers can produce the same kind of metadata that the type library importer produces, so you can create an Interop Assembly directly from .NET source code. This advanced technique is covered in Chapter 21, "Manually Defining COM Types in Source Code." An easy solution to a lack of type information is to perform late binding, if the COM objects you wish to use support the IDispatch interface. See the "Common Interactions with COM Objects" section for more information.

Primary Interop Assemblies

The previously described process of generating Interop Assemblies has an undesirable side effect due to the differences in the definition of identity between COM and the .NET Framework. Therefore, extra support exists to map .NET and COM identities more appropriately.

In COM, a type is identified by its GUID. It doesn't matter where you get the definition; if the numeric value of the GUID is correct, then everything works. You might find a common COM interface (such as IFont) defined in ten different type libraries, rather than each library referencing the official definition in the OLE Automation type library (STDOLE2.TLB). This duplication doesn't matter in the world of COM.

On the other hand, if ten different assemblies each have a definition of IFont, these are considered ten unrelated interfaces in managed code simply because they reside in different assemblies. The containing assembly is part of a managed type's identity, so it doesn't matter if multiple interfaces have the same name or look identical in all other respects.

This is the root of the identity problem. Suppose ten software companies write .NET applications that use definitions in the OLE Automation type library (STDOLE2.TLB). Each company uses Visual Studio .NET and adds a reference to the type library, causing a new assembly to be generated called stdole.dll. Each company digitally signs the assemblies that comprise the application, including the stdole Interop Assembly, as shown in Figure 3.6.

Figure 3.6 Applications from ten different companies—each using their own Interop Assembly for the OLE Automation type library.

Now imagine a user's computer with all ten of these programs installed. One problem is that the Global Assembly Cache is cluttered with Interop Assemblies that have no differences except for the publisher's identity, shown in Figure 3.7. The difference in publishers can be seen as differences in the public key tokens.

Figure 3.7 The opposite of DLL Hell—a cluttered Global Assembly Cache with multiple Interop Assemblies for the same type library.

Even if all these Interop Assemblies aren't installed in the GAC, the computer could be cluttered in other places, such as local application directories. This is the opposite of DLL Hell—application isolation taken to an extreme. There is no sharing whatsoever, not even when it's safe for the applications to do so.

Besides clutter, the main problem is that none of these applications can easily communicate as they could if they were unmanaged COM applications. If the Payroll assembly from Figure 3.6 exposes a method with an IFont parameter and the Search assembly wants to call this, it would try passing a stdole.IFont signed by Donna's Antiques. This doesn't work because the method expects a stdole.IFont signed by Rick's Aviation. As far as the CLR is concerned, these are completely different types.

What we really want is a single managed identity for a COM component's type definitions. Such "blessed" Interop Assemblies do exist, and are known as Primary Interop Assemblies (PIAs). A Primary Interop Assembly is digitally signed by the publisher of the original COM component. In the case of the OLE Automation type library, the publisher is Microsoft. A Primary Interop Assembly is not much different from any other Interop Assembly. Besides being digitally signed by the COM component's author, it is marked with a PIA-specific custom attribute (System.Runtime.InteropServices.PrimaryInteropAssemblyAttribute), and usually registered specially.

Figure 3.8 shows what the ten applications would look like if each used the Primary Interop Assembly for the OLE Automation Type Library rather than custom Interop Assemblies.

Figure 3.8 Applications from ten different companies—each using the Primary Interop Assembly for the OLE Automation type library.

Nothing forces .NET applications to make use of a PIA when one exists. The notion of Primary Interop Assemblies is just a convention that can be used by tools, such as Visual Studio .NET, to help guide software developers in the right direction by referencing common types.

To make use of Primary Interop Assemblies, adding a reference to a type library in Visual Studio .NET is a little more complicated than what was first stated. Visual Studio .NET tries to avoid invoking the type library importer if at all possible, since it generates a new assembly with its own identity. Instead, Visual Studio .NET automatically adds a copy of a PIA to a project's references if one is registered for a type library that the user references on the COM tab of the Add Reference dialog. (A copy of an assembly is just as good as the original, since the internal assembly identity is the same.) The TLBIMP.EXE utility, on the other hand, simply warns the user when attempting to create an Interop Assembly for a type library whose PIA is registered on the current computer; it still creates a new Interop Assembly. TLBIMP.EXE does make use of registered PIAs for dependent type libraries, described in the next chapter.

As Primary Interop Assemblies are created for existing COM components, they will likely be available for download from MSDN Online or component vendor Web sites. At the time of writing, no PIAs other than the handful that ship with Visual Studio .NET exist.

Digging Deeper -

It's possible to work around the identity problem caused by multiple Interop Assemblies without a notion of Primary Interop Assemblies. Due to the way in which COM interfaces are handled by the CLR, it's possible to cast from a COM interface definition in one assembly to a COM interface definition in another assembly if they have the same IID. (This does not work for regular .NET interfaces.) It's also possible to convert from a COM class defined in one assembly to a COM class defined in another assembly using the Marshal.CreateWrapperOfType method. Nothing enables converting between instances of duplicate structure definitions, but a structure's fields could be copied one-by-one.

But Primary Interop Assemblies have another important use. Interop Assemblies often need modifications, such as the ones shown in Chapter 7 to be completely usable in managed code. When you have a PIA with such customizations registered on your computer, you can benefit from these customizations simply by referencing the type library for the COM component you wish to use inside Visual Studio .NET. For example, the PIA for Microsoft ActiveX Data Objects (ADO), which ships with Visual Studio .NET, contains some customizations to handle object lifetime issues. If you created your own Interop Assembly for ADO using TLBIMP.EXE, you would not benefit from these customizations.

The process of creating and registering Primary Interop Assemblies is discussed in Chapter 15, "Creating and Deploying Useful Primary Interop Assemblies."

Using COM Objects in ASP.NET Pages

As in ASP pages, COM objects can be created in ASP.NET Web pages with the <object> tag. Using the <object> tag with a runat="server" attribute enables the COM object to be used in server-side script blocks. The <object> tag can contain a class identifier (CLSID) or programmatic identifier (ProgID) to identify the COM component to instantiate. The following code illustrates how to create an instance of a COM component using a CLSID:

<object id="MyObject" runat="server"

And the following code illustrates how to create an instance of a COM component using a ProgID:

<object id="MyObject" runat="server"

These two COM-specific techniques use late binding and do not require Interop Assemblies in order to work. However, the ASP.NET <object> tag can also be used with a class attribute to instantiate .NET objects or COM objects described in an Interop Assembly. This can be used as follows:

<object id="recset" runat="server"
		 class="ADODB.RecordsetClass, ADODB, Version=7.0.3300.0, 
		 Culture=neutral,  _PublicKeyToken=b03f5f7f11d50a3a"

The class string contains an assembly-qualified class name, which has the ClassName, AssemblyName format. The assembly name could be just an assembly's simple name (such as ADODB or SpeechLib) if the assembly is not in the Global Assembly Cache. This is known as partial binding since the desired assembly isn't completely described. Assemblies that may reside in the Global Assembly Cache, however, must be referenced with their complete name, as shown in the preceding example. (Version policy could still be applied to cause a different version of the assembly to be loaded than the one specified.) As with the C++ Hello, World example, the Class suffix must be used with the class name.

Interop Assemblies used in ASP.NET pages should be placed with any other assemblies, such as in the site's \bin directory.

ASP.NET also provides several ways to create COM objects inside the server-side script block:

  • Using Server.CreateObject

  • Using Server.CreateObjectFromClsid

  • Using the new Operator

The Server.CreateObject method should be familiar to ASP programmers. Server.CreateObject is a method with one string parameter that enables you to create an object from its ProgID. The following code illustrates how to create an instance of a COM component using Server.CreateObject in Visual Basic .NET:

Dim connection As Object
connection = Server.CreateObject("ADODB.Connection")

The Server.CreateObjectFromClsid method is used just like Server.CreateObject but with a string representing a CLSID.

Again, these are two COM-specific mechanisms that use late binding. To take advantage of an Interop Assembly, you could use the new operator as in the Hello, World examples or use an overload of the Server.CreateObject method. This overload has a Type object parameter instead of a string. To use this method, we can obtain a Type object from a method such as System.Type.GetType, which is described in the "Common Interactions with COM Objects" section.

Tip -

Using Server.CreateObject with a Type object obtained from Type.GetType or using the <object> tag with the class attribute is the preferred method of creating a COM object in an ASP.NET page. Besides giving you the massive performance benefits of early binding, it supports COM objects that rely on OnStartPage and OnEndPage notifications as given by classic ASP pages (whereas the New keyword does not).

Example: Using ADO in ASP.NET

To demonstrate the use of COM components in an ASP.NET Web page, this example uses one of the most widely used COM components in ASP—Microsoft ActiveX Data Objects, or ADO.

Tip -

The functionality provided by ADO is available via ADO.NET, a set of data-related classes in the .NET Framework. Using these classes in the System.Data assembly is the recommended way to perform data access in an ASP.NET Web page.

However, because a learning curve is involved in switching to a new data-access model, you may prefer to stick with ADO. Thanks to COM Interoperability, you can continue to use these familiar COM objects when upgrading your Web site to use ASP.NET.

Listing 3.1 demonstrates using the ADO COM component in an ASP.NET page by declaring two ADO COM objects with the <object> tag.

Listing 3.1 Traditional ADO Can Be Used in an ASP.NET Page Using the Familiar <object> Tag

 1: <%@ Page aspcompat=true %>
 2: <%@ Import namespace="System.Data" %>
 3: <script language="VB" runat="server">
 4: Sub Page_Load(sender As Object, e As EventArgs)
 5:  Dim strConnection As String
 6:  Dim i As Integer
 8:  ' Connection string for the sample "pubs" database
 9:  strConnection = _ 
10:   "DRIVER={SQL Server};SERVER=(local);UID=sa;PWD=;DATABASE=pubs;"
12:  Try
13:   ' Call a method on the page's connection object
14:   connection.Open(strConnection)
15:  Catch ex as Exception
16:   Response.Write("Unable to open connection to database. " + ex.Message)
17:  End Try
19:  Try
20:   ' Set properties and call a method on the page's recordset object
21:   recordset.CursorType = 1 ' 1 = ADODB.CursorTypeEnum.adOpenKeyset
22:   recordset.LockType = 3  ' 3 = ADODB.LockTypeEnum.adLockOptimistic
23:   ' 2 = ADODB.CommandTypeEnum.adCmdTable
24:   recordset.Open("titles", connection, , , 2) 
25:  Catch ex as Exception
26:   Response.Write("Unable to open recordset. " + ex.Message)
27:  End Try
29:  Dim table As DataTable
30:  Dim row As DataRow
32:  ' Create a DataTable
33:  table = New DataTable()
35:  ' Add the appropriate columns
36:  For i = 0 to recordset.Fields.Count - 1
37:   table.Columns.Add(New DataColumn(recordset.Fields(i).Name, _
38:    GetType(String)))
39:  Next
41:  ' Scan through the recordset and add a row for each record
42:  Do While Not recordset.EOF
43:   row = table.NewRow()
45:   ' Look at each field and add an entry to the row
46:   For i = 0 to recordset.Fields.Count - 1
47:    row(i) = recordset.Fields(i).Value.ToString()
48:   Next
50:   ' Add the row to the DataTable
51:   table.Rows.Add(row)
53:   recordset.MoveNext()
54:  Loop
56:  ' Update the DataGrid control
57:  dataGrid1.DataSource = New DataView(table)
58:  dataGrid1.DataBind()
60:  ' Cleanup
61:  recordset.Close()
62:  connection.Close()
63: End Sub
64: </script>
66: <html><title>Using ADO in ASP.NET</title>
67:  <body>
68:   <form runat=server>
69:    <asp:DataGrid id="dataGrid1" runat="server"
70:     BorderColor="black"
71:     GridLines="Both"
72:     BackColor="#ffdddd"
73:    />
74:   </form>
75:   <object id="connection" runat="server"
76:    progid="ADODB.Connection"/>
77:   <object id="recordset" runat="server"
78:    progid="ADODB.Recordset"/>
79:  </body>
80: </html>

The <%@ Page aspcompat=true %> directive in Line 1 is explained in Chapter 6, "Advanced Topics for Using COM Components." The important part of Listing 3.1 is Lines 75–78, which declare the two COM objects used in the ASP.NET page via ProgIDs. The HTML portion of the page contains one control—the DataGrid control. This data grid will hold the information that we obtain using ADO.

Inside the Page_Load method, Lines 9–10 initialize a connection string for the sample database (pubs) in the local machine's SQL Server. You may need to adjust this string appropriately to run the example on your computer. Lines 21–24 use a few "magic numbers"—hard-coded values that represent various ADO enumeration values mentioned in the code's comments. If you use the Primary Interop Assembly for ADO, then you can reference the actually enum values instead.

After creating a new DataTable object in Line 33, we add the appropriate number of columns by looping through the Fields collection. On Line 37, each column added is given the name of the current field's Name property, and the type of the data held in each column is set to the type String. The Do While loop starting in Line 42 processes each record in the Recordset object. Once Recordset.EOF is true, we've gone through all the records. With each record, we create a new row (Line 43), add each of the record's fields to the row (Lines 46–48), and add the row to the table (Line 51). Each string added to a row is the current field's Value property. ToString is called on the Value property in Line 47, in case the data isn't already a string. In Lines 57–58, we associate the DataTable object with the page's DataGrid control and call DataBind to display the records. Finally, Lines 61–62 call Close on the two ADO objects to indicate that we're finished with them.

Figure 3.9 displays the output of Listing 3.1 as shown in a Web browser.

Figure 3.9 The output of Listing 3.1 when viewed in a Web browser.

Using COM+ Components

Just like COM components, COM+ components—formerly Microsoft Transaction Server (MTS) components—can be used in an ASP.NET application. However, due to differences between the ASP and ASP.NET security models, using COM+ components in an ASP.NET application might require changing their security settings.

If you get an error with a message such as "Permission denied" when attempting to use COM+ components, you should be able to solve the problem as follows:

  1. Open the Component Services explorer.

  2. Under the Component Services node, find the COM+ application you wish to use, right-click on it, and select Properties.

  3. Go to the Identity tab and change the account information to a brand new local machine account.

  4. At a command prompt, run DCOMCNFG.EXE, a tool that lets you configure DCOM settings (such as security) for your COM+ application.

  5. Go to the Default Security tab and click the Edit Default... button in the Default Access Permissions area.

  6. Add the new user created in Step 3.

  7. Restart Internet Information Services (IIS).

An Introduction to Interop Marshaling

When calls are made from managed code to unmanaged code (or vice-versa), data passed in parameters needs to be converted from one representation to another. These conversions are described in the next chapter. The process of packaging data to be sent from one entity to another is known as marshaling. In COM, marshaling is done when sending data across context boundaries such as apartments, operating system processes, or computers. (Apartments are discussed in Chapter 6.) To differentiate the marshaling performed by the CLR from COM marshaling, the process of marshaling across unmanaged/managed boundaries is known as Interop marshaling.

COM marshaling often involves extra work for users, such as registering a type library to be used by the standard OLE Automation marshaler or registering your own proxy/stub DLL to handle marshaling for custom interfaces that can't adequately be described in a type library. Interop marshaling, on the other hand, is handled transparently in a way that's usually sufficient by a CLR component known as the Interop Marshaler. The metadata inside an Interop Assembly gives the Interop Marshaler the information it needs to be able to marshal data from .NET to COM and vice-versa.

Interop marshaling is completely independent of COM marshaling. COM marshaling occurs externally to the CLR, just as in the pre-.NET days. If COM marshaling is needed due to a call to a COM component occurring across contexts, Interop marshaling occurs either before or after COM marshaling, depending on the order of data flow. One direction of COM marshaling and Interop marshaling is pictured in Figure 3.10.

Figure 3.10 The relationship between Interop marshaling and COM marshaling.

In this diagram, a COM component returns a COM string type known as a BSTR to a .NET application calling it. BSTR is a pointer to a length-prefixed Unicode string. The pictured string contains "abc" and has a four-byte length prefix that describes the string as six bytes long. (The extra zeros appear in memory because each Unicode character occupies two bytes.) The calling .NET object happens to be in a different apartment, so standard COM marshaling copies the string to the caller's apartment. See Chapter 6 for controlling what kind of apartment a .NET application runs in when using COM Interoperability.

Once the BSTR has been marshaled by a standard COM mechanism, the Interop Marshaler is responsible for copying the data to a .NET string instance (System.String). Although .NET strings are also Unicode and length-prefixed, they have additional information stored in their prefix. Hence, a copy must be made between these two bitwise-incompatible entities. This new System.String instance can then be returned to the .NET caller (after the Interop Marshaler calls SysFreeString to destroy the BSTR). The CLR and the Interop Marshaler don't know or care that COM marshaling has occurred before it performed Interop Marshaling, as long as the unmanaged data is presented correctly in the current context. If an interface pointer were being passed from one apartment to the next and the interface did not have an appropriate COM marshaler registered for it, the call would fail just as it would in a pure COM scenario.

Many data types, such as strings, require copying from one internal representation to another during an Interop transition. When such data types are used by-reference, the Interop Marshaler provides copy-in/copy-out behavior rather than true by-reference semantics. Several common data types, however, have the same managed and unmanaged memory representations. A data type with the same managed and unmanaged representation is known as blittable. The blittable .NET data types are:

  • System.SByte

  • System.Int16

  • System.Int32

  • System.Int64

  • System.IntPtr

  • System.Byte

  • System.UInt16

  • System.UInt32

  • System.UInt64

  • System.UIntPtr

  • System.Single

  • System.Double

  • Structs composed only of the previous types

  • C-style arrays whose elements are of the previous types

In version 1.0 of the CLR, the Interop Marshaler pins and directly exposes managed memory to unmanaged code for any blittable types. (Pinning is discussed in Chapter 6.) By taking advantage of a common memory representation, the Interop Marshaler copies data only when necessary and therefore exhibits better performance when blittable types are used. This implementation detail can show up in more ways than performance, however. For example, copying data in or out of an Interop call can be suppressed by custom attributes, but for blittable types these same custom attributes have no effect because the original memory is always directly exposed to the method being called. Chapter 12, "Customizing COM's View of .NET Components," discusses these custom attributes and their relationship to blittable data types.

You can customize the behavior of the Interop Marshaler in two ways:

  • Changing the metadata inside Interop Assemblies to change the way the Interop Marshaler treats data types. This technique is covered in Chapter 6.

  • Plugging in your own custom marshaler, as discussed in Chapter 20, "Custom Marshaling."

Common Interactions with COM Objects

Interacting with types defined inside an Interop Assembly often feels just as natural as interacting with .NET types. There are some additional options and subtleties, however, and they're discussed in this section.

Creating an Instance

Creating an instance of a COM class can be just like creating an instance of a .NET class. The Hello, World example showed the most common way of creating a COM object—using the new operator. At run time, after the metadata is located, the class's ComImportAttribute pseudo-custom attribute tells the Common Language Runtime to create an RCW for the class, and to construct the COM object by calling CoCreateInstance using the CLSID specified in the class's GuidAttribute.


When instantiating an RCW, the exact CoCreateInstance call that the new operator maps to is as follows, in C++ syntax:

    CoCreateInstance(clsid, NULL, CLSCTX_SERVER, IID_IUnknown, (void**)&punk);

The CLSCTX_SERVER flag means that the object could be created in-process, out-of-process, or even on a remote computer (using DCOM), depending on how the coclass is registered.

If you need to modify the behavior of this call, you could alternatively call CoCreateInstance or CoCreateInstanceEx yourself using Platform Invocation Services (PInvoke). This technique is shown in Chapter 19, "Deeper Into PInvoke and Useful Examples."

Not all coclasses are creatable, however. Instances of noncreatable types can only be obtained when returned from a method or property. The object's RCW is created as soon as you obtain a COM object in this way.

Alternatives to the new Operator

There are other ways to create a COM object, some of which don't even require metadata for the COM object. In Visual Basic 6, you can avoid the need to reference a type library by calling CreateObject and passing the object's ProgID (a string). In the .NET Framework, you can avoid the need to reference an Interop Assembly by using the System.Type and System.Activator classes.

Creating an object in this alternative way is a two-step process:

  1. Get a Type instance so that we can create an instance of the desired class. This can be done three different ways, shown here in C# using the Microsoft Speech SpVoice class:

    • Type t = Type.GetTypeFromProgID("SAPI.SpVoice");

    • Type t = Type.GetTypeFromCLSID(new Guid("96749377-3391-11D2-9EE3-00C04F797396"));

    • Type t = Type.GetType("SpeechLib.SpVoiceClass, SpeechLib, Version=, Culture=neutral, PublicKeyToken=null");

  2. Create an instance of the type (shown in C#):

				Object voice = Activator.CreateInstance(t);

In step 1, the goal of all three techniques is to return a Type instance whose GUID property is set to the CLSID of the COM object we want to create. The first technique obtains the CLSID indirectly from the registry by using the class's ProgID. Most COM classes are registered with ProgIDs (which doesn't necessarily match the type name qualified with the library name, as in this example) so calling GetTypeFromProgID is a popular option. The second technique passes the CLSID directly, which should be reserved for COM objects without a ProgID due to its lack of readability.

Whereas the first two techniques don't require an Interop Assembly (which is great for COM objects that aren't described in a type library), the third technique of using Type.GetType does require one at run time. That's because the string passed to Type.GetType represents a .NET type whose metadata description must be found and loaded in order for the call to succeed. The format for the string is the same as the string used with the class attribute for an <object> tag in an ASP.NET page:

ClassName, AssemblyName

Step 2 creates an instance of the COM object and assigns it to an Object variable. With this variable, you could call members in a late bound fashion, shown in the "Calling Methods and Properties on a COM Object" section. Or, if you compiled the program with metadata definitions of interfaces that the COM object implements, you could cast the object to such an interface. In the SpVoice example, you could do the following:

SpVoice voice = (SpVoice)Activator.CreateInstance(t);

However, if you compile with a metadata definition of SpVoice, then you might as well instantiate the object using new.

Note -

Type.GetTypeFromProgID and Type.GetTypeFromCLSID have overloads that accept a string parameter with a server name. This enables COM objects to be created on a specified remote computer even if they are registered to also be creatable locally. Chapter 6 has more information.

For backwards compatibility with Visual Basic 6, you can still call CreateObject in Visual Basic .NET. Calling this method with a ProgID does the same thing as calling Type.GetTypeFromProgID followed by Activator.CreateInstance. (As with any .NET APIs, this can be called from other languages as well. CreateObject is simply a static method on the Interaction class inside the Microsoft.VisualBasic assembly.)

Detecting Errors

The most common error when attempting to create a COM object is due to the class not being registered (meaning that the CLSID for the class doesn't have an entry under HKEY_CLASSES_ ROOT\CLSID in the Windows Registry). When an error occurs during object creation, a COMException (defined in System.Runtime.InteropServices) is thrown. When calling one of the three Type.GetType... methods, however, failure is indicated by returning a null Type object unless you call the overloaded method with a boolean throwOnError parameter.

Because the goal of calling Type.GetTypeFromCLSID is to return a Type instance with the appropriate GUID, and because the desired GUID is passed directly as a parameter, the implementation of Type.GetTypeFromCLSID doesn't bother checking the Windows Registry. Instead, it always returns a Type instance whose GUID property is set to the passed-in CLSID. Therefore, failure is not noticed until you attempt to instantiate the class using the returned Type instance.

Calling Methods and Properties on a COM Object

When you have metadata for the COM object, calling methods is no different from calling methods and properties on a .NET object (as shown in the Hello, World example):

voice.Speak("Hello, World!", SpeechVoiceSpeakFlags.SVSFDefault);

There is one thing to note about properties and C#. Sometimes COM properties aren't supported by C# because they have by-reference parameters or multiple parameters. (C# does support properties with multiple parameters, also known as parameterized properties, if they are also default properties.) In such cases, C# doesn't allow you to call these properties with the normal property syntax, but does allow you to call the accessor methods directly. An example of this can be seen with the Microsoft SourceSafe 6.0 type library, which has the following property defined on the IVSSDatabase interface (shown in IDL):

[id(0x00000008), propget]
HRESULT User([in] BSTR Name, [out, retval] IVSSUser** ppIUser);

In C#, this property must be called as follows:

user = database.get_User("Guest");

Attempting to call the User property directly would cause compiler error CS1546 with the following message:

Property or indexer 'User' is not supported by the language; try directly 
calling accessor method 'SourceSafeTypeLib.VSSDatabase.get_User(string)'.

In Visual Basic .NET, this same property can be used with regular property syntax:

user = database.User("Guest")

If you don't have metadata for the COM object (either by choice or because the COM object doesn't have a type library), you must make late-bound calls, as the compiler has no type definitions. In Visual Basic .NET, this looks the same as it did in Visual Basic 6 (as long as you ensure that Option Strict is turned off):

Imports System
Module Module1
 Sub Main()
  Dim t as Type
  Dim voice as Object
  t = Type.GetTypeFromProgID("SAPI.SpVoice")
  voice = Activator.CreateInstance(t)
  voice.Speak("Hello, World!")
 End Sub
End Module

This can simply be compiled from the command line as follows, which does not require the SpeechLib Interop Assembly:

vbc HelloWorld.vb

Because the type of voice is System.Object, the Visual Basic .NET compiler doesn't check the method call at compile time. It's possible that a method called Speak won't exist on the voice object at run time or that it will have a different number of parameters (although we know otherwise), but all failures are reported at run time when using late binding.

In C#, however, the equivalent code doesn't compile:

using System;

class Class1
 static void Main()
  Type t = Type.GetTypeFromProgID("SAPI.SpVoice");
  object voice = Activator.CreateInstance(t);

  // Causes compiler error CS0117: 
  // 'object' does not contain a definition for 'Speak'
  voice.Speak("Hello, World!", 0);

That's because the type of voice is System.Object and there's no method called Speak on this type. The only way to make a late-bound call in C# is to use the general .NET feature of reflection, introduced in Chapter 1. Therefore, the previous C# example can be changed to:

using System;
using System.Reflection;

class Class1
 static void Main()
  Type t = Type.GetTypeFromProgID("SAPI.SpVoice");
  object voice = Activator.CreateInstance(t);
  object [] args = new Object[2];
  args[0] = "Hello, World!";
  args[1] = 0;
  t.InvokeMember("Speak", BindingFlags.InvokeMethod, null, voice, args);

When reflecting on a COM object using Type.InvokeMember, the CLR communicates with the object through its IDispatch interface. Different binding flags can be used to control what kind of member is invoked. Internally, which binding flag is chosen affects the wFlags parameter that the CLR passes to IDispatch.Invoke. These binding flags are:

  • BindingFlags.InvokeMethod. The CLR passes DISPATCH_METHOD, which indicates that a method should be invoked.

  • BindingFlags.GetProperty. The CLR passes DISPATCH_PROPERTYGET, which indicates that a property's get accessor (propget) should be invoked.

  • BindingFlags.SetProperty. The CLR passes DISPATCH_PROPERTYPUT | DISPATCH_PROPERTYPUTREF, which indicates that either a property's set accessor (propputref) or let accessor (propput) should be invoked. If the property has both of these accessors, it's up to the object's IDispatch implementation to decide which to invoke.

  • BindingFlags.PutDispProperty. The CLR passes DISPATCH_PROPERTYPUT, which indicates that a property's let accessor (propput) should be invoked.

  • BindingFlags.PutRefDispProperty. The CLR passes DISPATCH_PROPERTYPUTREF, which indicates that a property's set accessor (propputref) should be invoked.

For more information about COM properties, see Chapter 4.

When you don't have metadata for a COM object, the amount of information that you can get through the reflection API is limited. In addition (unlike .NET objects), not all COM objects support late binding because COM objects might not implement IDispatch. If you try to use Type.InvokeMember with such a COM object, you'll get an exception with the message:

The COM target does not implement IDispatch.

For these types of objects, having metadata definitions of the types and using different reflection APIs (such as MemberInfo.Invoke) is a must because late binding isn't an option.

Note -

When reflecting on a COM object that is defined in an Interop Assembly, you can use reflection APIs such as MemberInfo.Invoke even if the object doesn't support IDispatch. All of the reflection APIs except Type.InvokeMember call through a COM object's v-table unless the interface you're invoking on happens to be a dispinterface.

If you're reflecting on a COM object that has no metadata, Type.InvokeMember is just about the only reflection functionality you can make use of. For example, calling Type.GetMethods on such an object returns the methods of the generic System.__ComObject class rather than the COM object's methods. Without metadata, there is no way to enumerate a COM object's methods using reflection.

Using Optional Parameters

Optional parameters, commonly used in COM components, are parameters that a caller might omit. Optional parameters enable callers to use shorter syntax when the default values of parameters are acceptable, such as the second parameter of the Speak method used previously.

Optional Parameters in COM

In IDL, a method with optional parameters looks like this:

HRESULT PrintItems([in, optional] VARIANT x, [in, optional] VARIANT y);

In Visual Basic 6, this method might be implemented as follows:

Public Sub PrintItems(Optional ByVal x as Variant, Optional ByVal y as Variant)
 If Not IsMissing(x) Then ReallyPrint x
 If Not IsMissing(y) Then ReallyPrint y
End Sub

The VARIANT type can represent a missing value (meaning that the caller didn't pass anything). A missing value can be determined in VB6 using the built-in IsMissing method, and can be determined in unmanaged C++ by checking for a VARIANT with type VT_ERROR and a value equal to DISP_E_PARAMNOTFOUND (defined in winerror.h):

// MessageText:
// Parameter not found.

Non-VARIANT types can also be optional if they have a default value, demonstrated by the following method:


HRESULT AddItem([in, optional, defaultvalue("New Entry")] BSTR name,
 [in, optional, defaultvalue(1)] short importance);

Visual Basic 6:

Public Sub AddItem(Optional ByVal name As String = "New Entry", 
 Optional ByVal importance As Integer = 1)

For parameters with default values, the method implementer doesn't check for a missing value because if the caller didn't pass a value, it looks to the method as if the caller passed the default value.

Optional Parameters in the .NET Framework

Optional parameters also exist in the .NET Framework, although support for them is not required by the CLS. This means that you can't count on taking advantage of optional parameters in all .NET languages—C# being a prime example. Because C# does not support optional parameters, it can become frustrating to use COM objects that make heavy use of them.

FAQ: Why doesn't C# support optional parameters? -

The C# designers decided not to support optional parameters because their implementation has the unfortunate consequence of emitting the default value into the caller's MSIL instructions as if the caller passed the value in source code. For example, in Visual Basic .NET, viewing the IL produced for the following method call

reveals that the code produced is equivalent to the code that would be produced if the programmer had written

     list.AddItem("New Entry", 1)

This could result in versioning headaches if a future version of a component changes the default values because the values are hard-coded in the client. One could argue that a component shouldn't change default values as they are part of a contract with a client. One could also argue that it doesn't matter if the default values change because the clients still get the same behavior for the values they're passing in. If the component did want clients to switch to a new default, however, it couldn't be achieved without recompiling every client. Besides, C# prefers to be explicit to avoid any confusion. The recommended alternative to achieve the same effect as optional parameters when designing .NET components is to use method overloading. So, a C# programmer should define the following methods to get the same functionality as the earlier AddItem method:

public void AddItem()
 AddItem("New Entry", 1);
public void AddItem(string name)
 AddItem(name, 1);
public void AddItem(short importance)
 AddItem("New Entry", importance);
public void AddItem(string name, short importance)
 // Real implementation}

This encapsulates the default values in the component's implementation. Note that method overloading isn't possible in COM because each method on an interface must have a unique name.

The type library importer preserves the optional marking on parameters in the metadata that it produces. So, optional parameters in COM look like optional parameters to the .NET languages that support them. In Visual Basic .NET, for example, calling a COM method with optional parameters works the same way as it does in Visual Basic 6, as demonstrated in the Visual Basic .NET Hello, World example.

Behind the scenes, the VB .NET compiler fills in each missing parameter with either the default value or a System.Type.Missing instance if no default value exists. When passed to unmanaged code, the CLR converts a System.Type.Missing type to COM's version of a missing type—a VT_ERROR VARIANT with the value DISP_E_PARAMNOTFOUND. You could explicitly pass the Type.Missing static field for each optional parameter, but this isn't necessary as the VB .NET compiler does it for you. In languages like C#, however, passing Type.Missing can be useful for "omitting" a parameter.

Consider the following method, shown in both IDL and Visual Basic 6:


HRESULT AddAnyItem([in, optional, defaultvalue("New Entry")] VARIANT name,
 [in, optional, defaultvalue(1)] VARIANT importance);

Visual Basic 6:

Public Sub AddAnyItem(Optional ByVal name As Variant = "New Entry", 
 Optional ByVal importance As Variant = 1)

This method can be called in Visual Basic .NET the following ways, which are all equivalent:

  • list.AddAnyItem()

  • list.AddAnyItem("New Entry")

  • list.AddAnyItem(, 1)

  • list.AddAnyItem("New Entry", 1)

  • list.AddAnyItem(Type.Missing, Type.Missing)

  • list.AddAnyItem("New Entry", Type.Missing)

  • list.AddAnyItem(Type.Missing, 1)

Because the C# compiler ignores the optional marking in the metadata, only the last four ways of calling AddAnyItem can be used in C#.

Note -

Passing System.Reflection.Missing.Value as a missing type also works, but there's no reason to choose it over passing System.Type.Missing. Both are static fields that are an instance of the System.Reflection.Missing type, so both reflection and COM Interoperability treat these fields the same way.

For non-VARIANT optional parameters, C# programs must explicitly pass a default value to get the default behavior. That's because the compiler won't allow you to pass Type.Missing where a string is expected, for example. If you're not sure what the default value for a parameter is, view the type library using OLEVIEW.EXE or the corresponding metadata using ILDASM.EXE. The metadata for the AddAnyItem method is shown here:

.method public newslot virtual instance void
 AddAnyItem([in][opt] object marshal( struct) name,
       [in][opt] object marshal( struct) importance) 
 runtime managed internalcall
 .custom instance void [mscorlib]
  System.Runtime.InteropServices.DispIdAttribute::.ctor(int32) =
  ( 01 00 00 00 03 60 00 00 )
 .param [1] = "New Entry"
 .param [2] = int16(0x0001)
} // end of method IList::AddItem

Note -

There is actually a way to avoid explicitly passing default values to non-VARIANT optional parameters in C#, and that's to use reflection. Because the reflection APIs force you to package parameters as arrays of Objects, you can always pass Type.Missing for any kind of optional parameter, and reflection does the right thing.

But there's one more wrinkle for C# programmers—optional VARIANT parameters that are passed by-reference. As discussed in the next chapter, a VARIANT passed by-reference looks like ref object in C#. Thus, you can't simply pass Type.Missing or ref Type.Missing because it's a static field. You need to pass a reference to a variable that has been set to Type.Missing. For example, to call the following method from the Microsoft Word type library:

VARIANT_BOOL CheckSpelling(
 [in] BSTR Word,
 [in, optional] VARIANT* CustomDictionary,
 [in, optional] VARIANT* IgnoreUppercase,
 [in, optional] VARIANT* MainDictionary,
 [in, optional] VARIANT* CustomDictionary2,
 [in, optional] VARIANT* CustomDictionary3,
 [in, optional] VARIANT* CustomDictionary4,
 [in, optional] VARIANT* CustomDictionary5,
 [in, optional] VARIANT* CustomDictionary6,
 [in, optional] VARIANT* CustomDictionary7,
 [in, optional] VARIANT* CustomDictionary8,
 [in, optional] VARIANT* CustomDictionary9,
 [in, optional] VARIANT* CustomDictionary10);

you need the following silly-looking C# code:

object missing = Type.Missing;
result = msWord.CheckSpelling(
 word,      // Word
 ref missing,   // CustomDictionary
 ref ignoreUpper, // IgnoreUppercase
 ref missing,   // AlwaysSuggest
 ref missing,   // CustomDictionary2
 ref missing,   // CustomDictionary3
 ref missing,   // CustomDictionary4
 ref missing,   // CustomDictionary5
 ref missing,   // CustomDictionary6
 ref missing,   // CustomDictionary7
 ref missing,   // CustomDictionary8
 ref missing,   // CustomDictionary9
 ref missing);  // CustomDictionary10

It's ugly, but it works. This sometimes comes as a surprise because people don't often think of [in] VARIANT* in IDL as being passed by-reference, as there's no [out] flag.

Releasing a COM Object

The Common Language Runtime handles reference counting of COM objects, so there is no need to call IUnknown.AddRef or IUnknown.Release in managed code. You simply create the object using the new operator, or one of the other techniques discussed, and allow the system to take care of releasing the object.

Leaving it up to the CLR to release a COM object can sometimes be problematic because it does not occur at a deterministic time. Once you're finished using an RCW, it becomes eligible for garbage collection but does not actually get collected until some later point in time. And the wrapped COM object doesn't get released until the RCW is collected.

Sometimes COM objects require being released at a specific point during program execution. If you need to control exactly when the Runtime-Callable Wrapper calls IUnknown.Release, you can call the static (Shared in VB .NET) Marshal.ReleaseComObject method in the System.Runtime.InteropServices namespace. For a COM object called obj, this method can be called in Visual Basic .NET as follows:

Dim obj As MyCompany.ComObject
' We're finished with the object.

Marshal.ReleaseComObject has different semantics than IUnknown.Release—it makes the CLR call IUnknown.Release on every COM interface pointer it wraps, making the instance unusable to managed code afterwards. Attempting to use an object after passing it to ReleaseComObject raises a NullReferenceException. See Chapter 6 for more information about eagerly releasing COM objects.

Tip -

In Visual Basic 6, a COM object could be immediately released by setting the object reference to Nothing (null):

     Set comObj = Nothing

In managed code, however, setting an object to Nothing or null only makes the original instance eligible for garbage collection; the object is not immediately released. For COM objects, this can be accomplished by calling ReleaseComObject instead of or in addition to the previous line of code. For both .NET and COM objects, this can be accomplished by calling System.GC.Collect and System.GC. WaitForPendingFinalizers after setting the object to Nothing or null.

Casting to an Interface (QueryInterface)

In COM, calling QueryInterface is the way to programmatically determine whether an object implements a certain interface. The equivalent of this in .NET is casting. (In Visual Basic .NET, casting is done with either the CType or DirectCast operators.)

When attempting to cast a COM type to another type, the CLR calls QueryInterface in order to ask the object if the cast is legal, unless the type relationship can be determined from metadata. Because COM classes aren't required to list all the interfaces they implement in a type library, a QueryInterface call might often be necessary. Figure 3.11 diagrams what occurs when a COM object (an RCW) is cast to another type in managed code. There's more to the story than what is described here, however, which is indicated with the ellipses in the figure. Chapter 5, "Responding to COM Events," will update this diagram with the complete sequence of events.

Figure 3.11 Casting a COM object to another type in managed code.

This relationship between casting and QueryInterface means that an InvalidCastException is thrown in managed code whenever a QueryInterface call fails. When using COM components, however, an InvalidCastException can be thrown in other circumstances that don't involve casting. Chapter 6 discusses some of the common problems that cause an InvalidCastException.

The following code snippets illustrate how various unmanaged and managed languages enable coercing an instance of the SpVoice type to the ISpVoice interface:

Unmanaged C++:

IUnknown* punk = NULL;
ISpVoice* pVoice = NULL;
HRESULT hresult = punk->QueryInterface(IID_ISpVoice, (void**)&pVoice);

Visual Basic 6:

Dim i as ISpVoice
Dim v as SpVoice
Set i = v


ISpVoice i;
SpVoice v;
i = (ISpVoice)v;

Visual Basic .NET:

Dim i as ISpVoice
Dim v as SpVoice
i = CType(v, ISpVoice)

Because casting a COM object is like calling QueryInterface, COM objects should not be cast to a class type; they should only be cast to an interface. Not all COM objects expose a mechanism to determine what their class type is; the only thing you can count on is determining what interfaces it implements. However, because the names that represent classes in COM (such as SpVoice) now represent interfaces, casting to one of these special interfaces works. It results in a QueryInterface call to the class's default interface. What does not always work is casting a COM object to a type that's represented as a class in metadata, such as SpVoiceClass.

Error Handling

As mentioned in the preceding chapter, the COM way of error handling is to return a status code called an HRESULT. To bridge the gap between the two models of handling errors, the CLR checks for a failure HRESULT after an invocation and, if appropriate, throws an exception for managed clients to catch. The type of the exception thrown is based on the returned HRESULT, and the contents of the exception can contain customized information if the COM object sets additional information via the IErrorInfo interface. The translation between IErrorInfo and a .NET exception is covered in Chapter 16, "COM Design Guidelines for Components Used by .NET Clients." The exception types thrown by the CLR for various HRESULT values are listed in Appendix C, "HRESULT to .NET Exception Transformations."

In .NET, the type of an exception is often the most important aspect of an exception that enables clients to programmatically take a course of action. Although the CLR transforms some often-used HRESULTs (such as E_OUTOFMEMORY) into system-supplied exceptions (such as System.OutOfMemoryException), many COM components define and use custom HRESULTs. Unfortunately, such HRESULTs cannot be transformed to nice-looking exception types by the CLR. There are two reasons for this:

  • Appropriate exception types specific to custom HRESULT values would need to be defined somewhere in an assembly.

  • The CLR would need a mechanism for transforming arbitrary HRESULT values into arbitrary exception types, and there's no way to provide this information. In other words, the HRESULT transformation list in Appendix C is nonextensible.

Any unrecognized failure HRESULT is transformed to a System.Runtime.InteropServices. COMException. This is probably one of the most noticeable seams in the nearly seamless interoperation of COM components. COMExceptions have a public ErrorCode property that contains the HRESULT value, making it possible to check exactly which HRESULT caused this generic exception. This is demonstrated by the following Visual Basic .NET code:

 Dim msWord As Object = new Word.Application()
Catch ex as COMException
 If (ex.ErrorCode = &H80040154)
  MessageBox.Show("Word is not registered.")
  MessageBox.Show("Unexpected COMException: " + ex.ToString())
 End If
Catch ex as Exception
 MessageBox.Show("Unexpected exception: " + ex.ToString())
End Try

Other exception types typically don't have a public member that enables you to see the HRESULT, but every exception does have a corresponding HRESULT stored in a protected property, called HResult. The motivation is that in the .NET Framework, checking for error codes should be a thing of the past, and replaced by checking for the type of exception.

Note -

There is usually no need to check the HRESULT value inside an arbitrary exception, but it is possible by calling the System.Runtime.InteropServices.Marshal. GetHRForException method. It can also be done using the System.Runtime.InteropServices.ErrorWrapper class, but that isn't its intent.

Caution -

COM methods may occasionally change the data inside by-reference parameters before returning a failure HRESULT. When such a method is called from managed code, however, the updated values of any by-reference parameters are not copied back to the caller before the .NET exception is thrown. This effect is only seen with non-blittable types because any changes that the COM method makes to by-reference blittable types directly change the original memory.

All this discussion overlooks the fact that an HRESULT doesn't just represent an error code. It can be a nonerror status code known a success HRESULT, identified by a severity bit set to zero. Success HRESULTs other than S_OK (the standard return value when there's no failure) are used much less often than failure HRESULTs, but can show up when using members of widely used interfaces. One common example is the IPersistStorage.IsDirty method, which returns either S_OK or S_FALSE, neither of which is an error condition.

HRESULTs are hidden from managed code, so there's no way to know what HRESULT is returned from a method or property unless it causes an exception to be thrown. One could imagine a SuccessException being thrown whenever a COM object returns an interesting success HRESULT, but throwing an exception slows down an application and thus should be reserved for exceptional situations. Chapter 7 demonstrates a way to expose HRESULTs in imported signatures in order to handle success HRESULTs.

Enumerating Over a Collection

Thanks to the transformations performed by the type library importer, enumerating over a collection exposed by a COM object is as simple as enumerating over a collection exposed by a .NET object. This occurs as long as the COM collection is exposed as a member with DISPID_NEW_ENUM that returns an IEnumVARIANT interface pointer, as discussed in the next chapter.

The following is a Visual Basic .NET code snippet that demonstrates enumeration over a collection exposed by the Microsoft Word type library. More of this is shown in the example at the end of this chapter.

Dim suggestions As SpellingSuggestions
Dim suggestion As SpellingSuggestion
suggestions = msWord.GetSpellingSuggestions("errror")

' Enumerate over the SpellingSuggestions collection
For Each suggestion In suggestions

Passing the Right Type of Object

When COM methods or properties have VARIANT parameters, they look like System.Object types to managed code. Passing the right type of Object so that the COM component sees the right type of VARIANT is usually straightforward. For example, passing a managed Boolean means the component sees a VARIANT containing a VARIANT_BOOL (a Boolean in VB6), and passing a managed Double means the component sees a VARIANT containing a double.

The tricky cases are the managed data types that have multiple unmanaged representations. Some common types used in COM no longer exist in the .NET Framework: CURRENCY, VARIANT, IUnknown, IDispatch, SCODE, and HRESULT. Therefore, as discussed further in Chapter 4, .NET Decimal types can be used to represent COM CURRENCY or DECIMAL types, .NET Object types can be used to represent COM VARIANT, IUnknown, or IDispatch types, and .NET integers can be used to represent COM SCODE or HRESULT types. If a COM signature had a CURRENCY parameter, you could simply pass a Decimal when early-binding to the corresponding method described in an Interop Assembly. The CLR would automatically transform it to a CURRENCY because the type library importer decorates such parameters a custom attribute that tells the CLR what the unmanaged data type is. With a VARIANT parameter, however, there needs to be some way to control whether the type passed looks like a DECIMAL (VT_DECIMAL) or a CURRENCY (VT_CY) to the COM component, and this information is not captured in the signature.

The solution for using a single .NET type to represent multiple COM types lies in some simple wrappers (not to be confused with RCWs or CCWs) defined in the System.Runtime. InteropServices namespace. There is a wrapper for each basic type that doesn't exist in the managed world:

  • CurrencyWrapper Used to make a Decimal look like a CURRENCY type when passed inside a VARIANT.

  • UnknownWrapper Used to make an Object look like an IUnknown interface pointer when passed inside a VARIANT.

  • DispatchWrapper Used to make an Object look like an IDispatch interface pointer when passed inside a VARIANT.

  • ErrorWrapper Used to make an integer or an Exception look like an SCODE when passed inside a VARIANT.

Listing 3.2 shows C# code that demonstrates how to use these wrappers to convey different VARIANT types when calling the following GiveMeAnything method (shown in its unmanaged and managed representations):


HRESULT GiveMeAnything([in] VARIANT v);

Visual Basic 6:

Public Sub GiveMeAnything(ByVal v As Variant)


public virtual void GiveMeAnything(Object v);

Listing 3.2 Using CurrencyWrapper, UnknownWrapper, DispatchWrapper, and ErrorWrapper to Convey Different VARIANT Types

Decimal d = 123.456M;
int i = 10;
Object o = ...

// Pass a VARIANT with type VT_DECIMAL (Decimal)

// Pass a VARIANT with type VT_CY (Currency)
comObj.GiveMeAnything(new CurrencyWrapper(d));

// Pass a VARIANT with type VT_UNKNOWN
comObj.GiveMeAnything(new UnknownWrapper(o));

// Pass a VARIANT with type VT_DISPATCH
comObj.GiveMeAnything(new DispatchWrapper(o));

// Pass a VARIANT with whatever the type of the object is.
// For example, a String results in type VT_BSTR, and an object like
// System.Collections.Hashtable results in type VT_DISPATCH.

// Pass a VARIANT with type VT_I4 (long in IDL, Short in VB6)

// Pass a VARIANT with type VT_ERROR (SCODE)
comObj.GiveMeAnything(new ErrorWrapper(i));

// Pass a VARIANT with type VT_ERROR (SCODE) 
// using the value of the exception's internal HRESULT.
comObj.GiveMeAnything(new ErrorWrapper(new StackOverflowException()));

Caution -

The Interop Marshaler never creates wrapper types such as CurrencyWrapper when marshaling an unmanaged data type to a managed data type; these wrappers work in one direction only. Combined with the copy-in/copy-out semantics of by-reference parameters passed across an Interop boundary, this fact can cause behavior that sometimes surprises people. If you pass a CurrencyWrapper instance by-reference to unmanaged code (via a parameter typed as System.Object), it becomes a Decimal instance after the call even if the COM object did nothing with the parameter.

Table 3.1 summarizes what kind of instance can be passed as a System.Object parameter in order to get the desired VARIANT type when marshaled to unmanaged code. When early binding to a COM object, these only apply when the parameter type is System.Object, because otherwise these types would not be marshaled as VARIANTs.

Table 3.1 .NET Types and Their Marshaling Behavior Inside VARIANTs

Type of Instance


null (Nothing in VB .NET)
















































Any Array


System.Object or other .NET classes


An array of strings appears as VT_BSTR | VT_ARRAY, an array of doubles appears as VT_R8 | VT_ARRAY, and so on. The VT_I8 and VT_U8 VARIANT types listed in Table 3.1 are not supported prior to Windows XP. Also, notice that in version 1.0 the Interop Marshaler does not support VARIANTs with the VT_RECORD type (used for user-defined structures).

Tip -

Because null (Nothing) is mapped to an "empty object" (a VARIANT with type VT_EMPTY) when passed to COM via a System.Object parameter, you can pass new DispatchWrapper(null) or new UnknownWrapper(null) to represent a null object. This maps to a VARIANT with type VT_DISPATCH or VT_UNKNOWN, respectively, whose pointer value is null.

Digging Deeper -

The conversions in Table 3.1 are based on the type code that each type's IConvertible implementation returns from its GetTypeCode method. Therefore, any .NET object that implements IConvertible can control how it gets marshaled inside a VARIANT. For example, an object that returns a type code equal to TypeCode.Double is marshaled as a VT_R8 VARIANT.

Late Binding and By-Reference Parameters

Previous code examples have shown how to late bind to a COM component using either reflection or Visual Basic .NET late binding syntax. Table 3.1 and the use of wrappers such as CurrencyWrapper are also important for late binding because all parameters are packaged in VARIANTs when late binding to a COM component. One remaining issue that needs to be addressed is the handling of by-reference parameters.

When late binding to .NET members (or members defined in an Interop Assembly), the metadata tells reflection whether a parameter is passed by value or by reference. When late binding to COM members via IDispatch (using Type.InvokeMember in managed code), however, the CLR has no way to know which parameters must be passed by value and which must be passed by reference. Because all parameters are packaged in VARIANTs when late binding to a COM component, a by-reference parameter looks like a VARIANT whose type is bitwise-ORed with the VT_BYREF flag.

Reflection and the VB .NET late binding abstraction choose different default behavior when late binding to COM members—Visual Basic .NET passes all parameters by reference by default, but Type.InvokeMember passes all parameters by value by default!

Changing the default behavior in the VB .NET case is easy. As in earlier versions of Visual Basic, you can surround an argument in extra parentheses to force it to be passed by value:

Dim s As String = "SomeString"
' Call COM method via late binding
' The String parameter is passed by-reference (VT_BSTR | VT_BYREF)

' Call COM method again via late binding
' The String parameter is passed by-value (VT_BSTR)

Changing the default behavior with Type.InvokeMember is not so easy. To pass any parameters by reference, you must call an overload of Type.InvokeMember that accepts an array of System.Reflection.ParameterModifier types. You must pass an array with only a single ParameterModifier element that is initialized with the number of parameters in the member being invoked. ParameterModifier has a default property called Item (exposed as an indexer in C#) that can be indexed from 0 to NumberOfParameters-1. Each element in this property must either be set to true if the corresponding parameter should be passed by reference, or false if the corresponding parameter should be passed by value. This is summarized in Listing 3.3, which contains C# code that late binds to a COM object's method and passes a single string parameter first by reference and then by value.

Listing 3.3 Using System.Reflection.ParameterModifier to Pass Parameters to COM with the VT_BYREF Flag

using SomeComLibrary;
using System.Reflection;

public class LateBinding
 public static void Main()
  SomeComObject obj = new SomeComObject();

  object [] args = { "SomeString" };

  // Initialize a ParameterModifier with the number of parameters
  ParameterModifier p = new ParameterModifier(1);
  // Set the VT_BYREF flag on the first parameter
  p[0] = true;
  // Always create an array of ParameterModifiers with a single element
  ParameterModifier [] mods = { p };

  // Call the method via late binding. 
  // The parameter is passed by reference (VT_BSTR | VT_BYREF)
  obj.GetType().InvokeMember("SomeMethod", BindingFlags.InvokeMethod, null,
   obj, args, mods, null, null);

  // Call the method again using the simplest InvokeMember overload
  // The parameter is passed by value (VT_BSTR) 
  obj.GetType().InvokeMember("SomeMethod", BindingFlags.InvokeMethod, null,
   obj, args);

Using ParameterModifier is only necessary when reflecting using Type.InvokeMember because all other reflection methods use metadata that completely describes every parameter in the member being called. Note that there is no built-in way to pass an object to COM that appears as a VARIANT with the type VT_VARIANT | VT_BYREF. Additionally, in an early bound call, there is no way to pass a VARIANT with the VT_BYREF flag set without resorting to do-it-yourself marshaling techniques described in Chapter 6.

Using ActiveX Controls in .NET Applications

The terms ActiveX control and COM component are often used interchangeably, but in this book an ActiveX control is a special kind of COM component that supports being hosted in an ActiveX container. Such objects typically implement several interfaces such as IOleObject, IOleInPlaceObject, IOleControl, IDataObject, and more. They are typically registered specially as a control (as opposed to a simple class) and are marked with the [control] IDL attribute in their type libraries. They also usually have a graphical user interface.

The .NET equivalent of ActiveX controls are Windows Forms controls. Just as it's possible to expose and use COM objects as if they are .NET objects, it's possible to expose and use ActiveX controls as if they are Windows Forms controls. First we'll look at the process of referencing an ActiveX control in Visual Studio .NET, then how to do it with a .NET Framework SDK utility. Finally, we'll look at an example of hosting and using the control on a .NET Windows Form. For these examples, we'll use the WebBrowser control located in the Microsoft Internet Controls type library (SHDOCVW.DLL).

Referencing an ActiveX Control in Visual Studio .NET

If you referenced the Microsoft Internet Controls type library as you would any other type library, then the WebBrowser class could only be used like an ordinary object; you wouldn't be able to drag and drop it on a form. Using ActiveX controls as Windows Forms controls requires these steps:

  1. Start Visual Studio .NET and create either a new Visual Basic .NET Windows application or a Visual C# Windows application. The remaining steps apply to either language.

  2. Select Tools, Customize Toolbox... from the menu, or right-click inside the Toolbox window and select Customize Toolbox... from the context menu. There are two kinds of controls that can be referenced: COM Components and .NET Framework Components. The default COM Components tab shows a list of all the registered ActiveX controls on the computer. This dialog is shown in Figure 3.12.

  3. Select the desired control, then click the OK button.

Figure 3.12 Adding a reference to an ActiveX control in Visual Studio .NET.

If these steps succeeded, then an icon for the control should appear in the Toolbox window. Select it and drag an instance of the control onto your form just as you would with any Windows Forms control. At this point, at least two assemblies are added to your project's references—an Interop Assembly for the ActiveX control's type library (and any dependent Interop Assemblies), and an ActiveX Assembly that wraps any ActiveX controls inside the type library as special Windows Forms controls. This ActiveX Assembly always has the same name as the Interop Assembly, but with an Ax prefix. Figure 3.13 shows these two assemblies that appear in the Solution Explorer window when referencing and using the WebBrowser ActiveX control.

Figure 3.13 The Solution Explorer after an ActiveX control has been added to a Windows Form.

Referencing an ActiveX Control Using Only the .NET Framework SDK

Now, let's look at how to accomplish the same task using only the .NET Framework SDK. The following example uses the .NET ActiveX Control to Windows Forms Assembly Generator (AXIMP.EXE), also known as the ActiveX importer. This utility is the TLBIMP.EXE of the Windows Forms world, and can be used as follows:

  1. From a command prompt, type the following (replacing the path with the location of SHDOCVW.DLL on your computer):

         AxImp C:\Windows\System32\shdocvw.dll

    This produces both an Interop Assembly (SHDocVw.dll) and ActiveX Assembly (AxSHDocVw.dll) for the input type library in the current directory. Unlike TLBIMP.EXE, AXIMP.EXE does not search for the input file using the PATH environment variable.

  2. Reference the ActiveX Assembly just as you would any other assembly, which depends on the language. Depending on the nature of your application, you might also have to reference the Interop Assembly, the System.Windows.Forms assembly, and more.

The Interop Assembly created by AXIMP.EXE is no different from the one created by TLBIMP.EXE. If a Primary Interop Assembly for the input type library is registered on the current computer, AXIMP.EXE references that assembly rather than generating a new one.

If no ActiveX control can be found in an input type library, AXIMP.EXE reports:

AxImp Error: Did not find any registered ActiveX control in '...'.

In order for AXIMP.EXE to recognize a COM class as an ActiveX control, it must be registered on the current computer with the following registry value:


Being marked in the type library with the [control] attribute is irrelevant.

Example: A Simple Web Browser

Now that we know how to generate and reference an ActiveX Assembly that wraps an ActiveX control as a Windows Forms control, we'll put together a short example that uses an ActiveX control in managed code. Listing 3.4 demonstrates the use of the WebBrowser control to create a simple Web browser application, pictured in Figure 3.14. Parts of the listing are omitted, but the complete source code is available on this book's Web site.

Figure 3.14 The simple Web browser.

Listing 3.4 MyWebBrowser.cs. Using the WebBrowser ActiveX Control in C#

 1: using System;
 2: using SHDocVw;
 3: using System.Windows.Forms;
 5: public class MyWebBrowser : Form
 6: {
 7:  ...
 8:  object m = Type.Missing;
10:  // Constructor
11:  public MyWebBrowser()
12:  {
13:   // Required for Windows Form Designer support
14:   InitializeComponent();
15:   // Start on the home page
16:   axWebBrowser1.GoHome();
17:  }
19:  // Clean up any resources being used
20:  protected override void Dispose( bool disposing )
21:  {
22:   if (disposing)
23:   {
24:    if (components != null) 
25:    {
26:     components.Dispose();
27:    }
28:   }
29:   base.Dispose(disposing);
30:  }
32:  // Required method for Designer support
33:  private void InitializeComponent()
34:  {
35:   ...
36:   this.axWebBrowser1 = new AxSHDocVw.AxWebBrowser();
37:   ...
38:   ((System.ComponentModel.ISupportInitialize)
39:    (this.axWebBrowser1)).BeginInit();
40:   ... 
41:   this.axWebBrowser1.OcxState = ((System.Windows.Forms.AxHost.State)
42:    (resources.GetObject("axWebBrowser1.OcxState")));
43:   ...
44:   ((System.ComponentModel.ISupportInitialize)
45:    (this.axWebBrowser1)).EndInit();
46:   ...
47:  }
49:  [STAThread]
50:  static void Main() 
51:  {
52:   Application.Run(new MyWebBrowser());
53:  }
55:  // Called when one of the toolbar buttons is clicked
56:  private void toolBar1_ButtonClick(object sender, 
57:   ToolBarButtonClickEventArgs e)
58:  {
59:   if (e.Button.Text == "Back")
60:   {
61:    try { axWebBrowser1.GoBack(); }
62:    catch {}
63:   }
64:   else if (e.Button.Text == "Forward")
65:   {
66:    try { axWebBrowser1.GoForward(); }
67:    catch {}
68:   }
69:   else if (e.Button.Text == "Stop")
70:   {
71:    axWebBrowser1.Stop();
72:   }
73:   else if (e.Button.Text == "Refresh")
74:   {
75:    axWebBrowser1.CtlRefresh();
76:   }
77:   else if (e.Button.Text == "Home")
78:   {
79:    axWebBrowser1.GoHome();
80:   }
81:  }
83:  // Called when "Go" is clicked
84:  private void goButton_Click(object sender, System.EventArgs e)
85:  {
86:   axWebBrowser1.Navigate(navigateBox.Text, ref m, ref m, ref m, ref m);
87:  }
88: }

Line 8 declares a Missing instance used for optional parameters in Line 86. The constructor in Lines 11–17 first calls the standard InitializeComponent method to initialize the form's user interface, then calls GoHome on the ActiveX control to browse to the user's home page.

Lines 33–47 contain a few of the lines inside InitializeComponent that relate to the ActiveX control. Although the class is called WebBrowser, the class created by the ActiveX importer always begins with an Ax prefix. Therefore, Line 36 instantiates a new AxWebBrowser object. Lines 56–81 contain the event handler that gets called whenever the user clicks on one of the buttons across the top of the form. Whenever the Back and Forward buttons are clicked, the GoBack and GoForward methods are called, respectively. Because these methods throw an exception if there is no page to move to, any exception is caught and ignored. This is not the ideal way to implement these buttons, but it will have to wait until Chapter 5.

Notice that Line 75 calls a method called CtlRefresh, although the original WebBrowser control doesn't have such a method. What happens here is that any class created by the ActiveX importer ultimately derives from System.Windows.Forms.Control, and this class already has a property called Refresh. To distinguish members of the ActiveX control from members of the wrapper's base classes, the ActiveX importer places a Ctl prefix (which stands for control) on any members with conflicting names. The AxWebBrowser class has many other renamed members due to name conflicts—CtlContainer, CtlHeight, CtlLeft, CtlParent, CtlTop, CtlVisible, and CtlWidth.

Finally, Lines 84–87 call the ActiveX control's Navigate method when the user clicks the Go button.

Deploying a .NET Application That Uses COM

Deploying a .NET application that uses COM components is not quite as simple as deploying a .NET application that doesn't. Besides satisfying the requirements of the .NET Framework, you must satisfy the requirements of COM. This means registering the COM component(s) in the Windows Registry on the user's machine, just as you would have if no managed code were involved. This is usually accomplished by running REGSVR32.EXE on each COM DLL. It might also be necessary to register type libraries, which adds additional registry entries for interfaces. If you're relying on a component being installed (such as the Microsoft Speech SDK), no additional work is necessary, except the supplied installation.

Unless you late bind to the COM components and only create COM types via ProgID or CLSID, you also need to deploy metadata for the COM components you use. This is no different from managed types because metadata is needed at compile time and at run time. It sometimes seems like more of a burden for COM types, however, because the metadata resides in an Interop Assembly separate from the file containing the implementation. These should be installed just like other assemblies, either in the Global Assembly Cache and/or in an application-specific directory.

You should avoid deploying Interop Assemblies for COM components you didn't author if Primary Interop Assemblies already exist. If you need to create Primary Interop Assemblies for your own COM components, see Chapter 15.

Example: Using Microsoft Word to Check Spelling

To end the chapter, let's apply everything we've learned to a larger example. This example application is a very simple word processor, shown in Figure 3.15, which uses Microsoft Word for its spellchecker functionality. The user can type inside the application, and then click the Check Spelling button. Each misspelled word (according to Microsoft Word) is highlighted in red and underlined. At any time, the user can right-click a word and choose from a list of correctly spelled replacements supplied by Microsoft Word. The application also gives an option to ignore words with all uppercase letters. When selected, such a word isn't ever marked as misspelled, and alternate spellings aren't suggested when right-clicking it.

Figure 3.15 The example word processor.

The code, shown in C# in Listing 3.5, demonstrates the use of by-reference optional parameters, error handling with COM objects, and enumerating over a collection. The C# version is shown because calling the methods with optional parameters requires extra work. The Visual Basic .NET version on this book's Web site looks much less messy when calling these methods.

To compile or run this example, you must have Microsoft Word on your computer. The sample uses Word 2002, which ships with Microsoft Office XP. If you have a different version of Word installed, it should still work (as long as you use the appropriate type library instead of the one mentioned in step 2).

If you have Visual Studio .NET, here are the steps for creating and running this application:

  1. Create a new Visual C# Windows Application project.

  2. Add a reference to Microsoft Word 10.0 Object Library using the method explained at the beginning of this chapter.

  3. View the code for Form1.cs in your project, and change its contents to the code in Listing 3.5. One way to view the code is to right-click on the filename in the Solution Explorer window and select View Code.

  4. Build and run the project.

Otherwise, if all you have is the .NET Framework SDK, you can perform the following steps:

  1. Create and save a Form1.cs file with the code in Listing 3.5. The Windows Forms code inside InitializeComponent is omitted, but the complete source code is available on this book's Web site.

  2. Use TLBIMP.EXE to generate an Interop Assembly for the Microsoft Word type library as follows:

         TlbImp "C:\Program Files\Microsoft Office\Office10\msword.olb"

    The path for the input file may need to change depending on your computer's settings. If a PIA for the Word type library is available, you should download it from MSDN Online and use that instead of running TLBIMP.EXE.

  3. Compile the code, referencing all the needed assemblies:

    csc /t:winexe Form1.cs /r:Word.dll /r:System.Windows.Forms.dll 
    				/r:System.Drawing.dll /r:System.dll
  4. Run the generated executable.

Listing 3.5 Form1.cs. Using Microsoft Word Spell Checking in C#

 1: using System;
 2: using System.Drawing;
 3: using System.Windows.Forms;
 5: public class Form1 : Form
 6: {
 7:  // Visual controls
 8:  private RichTextBox richTextBox1;
 9:  private Button button1;
 10:  private CheckBox checkBox1;
 11:  private ContextMenu contextMenu1;
 13:  // Required designer variable
 14:  private System.ComponentModel.Container components = null;
 16:  // The Word application object
 17:  private Word.Application msWord;
 19:  // Two fonts used to display normal words and misspelled words
 20:  private Font normalFont = new Font("Times New Roman", 12);
 21:  private Font errorFont = new Font("Times New Roman", 12, 
 22:   FontStyle.Underline);
 24:  // Objects that need to be passed by-reference when calling Word
 25:  private object missing = Type.Missing;
 26:  private object ignoreUpper;
 28:  // Event handler used for the ContextMenu when 
 29:  // the user clicks on spelling suggestions
 30:  private EventHandler menuHandler;
 32:  // Constructor
 33:  public Form1()
 34:  {
 35:   // Required for Windows Form Designer support
 36:   InitializeComponent();
 37:   menuHandler = new System.EventHandler(this.Menu_Click);
 38:  }
 40:  // Called when the form is loading. Initializes Microsoft Word.
 41:  protected override void OnLoad(EventArgs e)
 42:  {
 43:   base.OnLoad(e);
 45:   try
 46:   {
 47:    msWord = new Word.Application();
 49:    // Call this in order for GetSpellingSuggestions to work later
 50:    msWord.Documents.Add(ref missing, ref missing, ref missing, 
 51:     ref missing);
 52:   }
 53:   catch (System.Runtime.InteropServices.COMException ex)
 54:   {
 55:    if ((uint)ex.ErrorCode == 0x80040154)
 56:    {
 57:     MessageBox.Show("This application requires Microsoft Word " +
 58:      "to be installed for spelling functionality. " +
 59:      "Since Word can't be located, the spelling functionality " +
 60:      "is disabled.", "Warning");
 61:     button1.Enabled = false;
 62:     checkBox1.Enabled = false;
 63:    }
 64:    else
 65:    {
 66:     MessageBox.Show("Unexpected initialization error. " + 
 67:      ex.Message + "\n\nDetails:\n" + ex.ToString(), 
 68:      "Unexpected Initialization Error", MessageBoxButtons.OK, 
 69:      MessageBoxIcon.Error);
 70:     this.Close();
 71:    }
 72:   }
 73:   catch (Exception ex)
 74:   {
 75:    MessageBox.Show("Unexpected initialization error. " + ex.Message +
 76:     "\n\nDetails:\n" + ex.ToString(), 
 77:     "Unexpected Initialization Error", MessageBoxButtons.OK, 
 78:     MessageBoxIcon.Error);
 79:    this.Close();
 80:   }
 81:  }
 83:  // Clean up any resources being used
 84:  protected override void Dispose(bool disposing)
 85:  {
 86:   if (disposing)
 87:   {
 88:    if (components != null) 
 89:    {
 90:     components.Dispose();
 91:    }
 92:   }
 93:   base.Dispose(disposing);
 94:  }
 96:  // Required method for Designer support
 97:  private void InitializeComponent()
 98:  {
 99:   ...
100:  }
102:  // The main entry point for the application
103:  [STAThread]
104:  public static void Main()
105:  {
106:   Application.Run(new Form1());
107:  }
109:  // Checks the spelling of the word contained in the string argument.
110:  // Returns true if spelled correctly, false otherwise.
111:  // This method ignores words in all uppercase letters if checkBox1 
112:  // is checked.
113:  private bool CheckSpelling(string word)
114:  {
115:   ignoreUpper = checkBox1.Checked;
117:   // Pass a reference to Type.Missing for each 
118:   // by-reference optional parameter
120:   return msWord.CheckSpelling(word, // Word
121:    ref missing,          // CustomDictionary
122:    ref ignoreUpper,        // IgnoreUppercase
123:    ref missing,          // AlwaysSuggest
124:    ref missing,          // CustomDictionary2
125:    ref missing,          // CustomDictionary3
126:    ref missing,          // CustomDictionary4
127:    ref missing,          // CustomDictionary5
128:    ref missing,          // CustomDictionary6
129:    ref missing,          // CustomDictionary7
130:    ref missing,          // CustomDictionary8
131:    ref missing,          // CustomDictionary9
132:    ref missing);          // CustomDictionary10
133:  }
135:  // Checks the spelling of the word contained in the string argument.
136:  // Returns a SpellingSuggestions collection, which is empty if the word
137:  // is spelled correctly.
138:  // This method ignores words in all uppercase letters if checkBox1
139:  // is checked.
140:  private Word.SpellingSuggestions GetSpellingSuggestions(string word)
141:  {
142:   ignoreUpper = checkBox1.Checked;
144:   // Pass a reference to Type.Missing for each 
145:   // by-reference optional parameter
147:   return msWord.GetSpellingSuggestions(word, // Word
148:    ref missing,               // CustomDictionary
149:    ref ignoreUpper,             // IgnoreUppercase
150:    ref missing,               // MainDictionary
151:    ref missing,               // SuggestionMode
152:    ref missing,               // CustomDictionary2
153:    ref missing,               // CustomDictionary3
154:    ref missing,               // CustomDictionary4
155:    ref missing,               // CustomDictionary5
156:    ref missing,               // CustomDictionary6
157:    ref missing,               // CustomDictionary7
158:    ref missing,               // CustomDictionary8
159:    ref missing,               // CustomDictionary9
160:    ref missing);              // CustomDictionary10
161:  }
163:  // Called when the "Check Spelling" button is clicked.
164:  // Checks the spelling of each word and changes the font of 
165:  // each misspelled word.
166:  private void button1_Click(object sender, EventArgs e)
167:  {
168:   try
169:   {
170:    // Return all text to normal since 
171:    // underlined spaces might be left behind
172:    richTextBox1.SelectionStart = 0;
173:    richTextBox1.SelectionLength = richTextBox1.Text.Length;
174:    richTextBox1.SelectionFont = normalFont;
176:    // Beginning location in the RichTextBox of the next word to check
177:    int index = 0;
179:    // Enumerate over the collection of words obtained from String.Split
180:    foreach (string s in richTextBox1.Text.Split(null))
181:    {
182:     // Select the word
183:     richTextBox1.SelectionStart = index;
184:     richTextBox1.SelectionLength = s.Length;
186:     // Trim off any ending punctuation in the selected text
187:     while (richTextBox1.SelectionLength > 0 && 
188:      Char.IsPunctuation(
189:      richTextBox1.Text[index + richTextBox1.SelectionLength - 1]))
190:     {
191:      richTextBox1.SelectionLength--;
192:     }
194:     // Check the word's spelling
195:     if (!CheckSpelling(s))
196:     {
197:      // Mark as incorrect
198:      richTextBox1.SelectionFont = errorFont;
199:      richTextBox1.SelectionColor = Color.Red;
200:     }
201:     else
202:     {
203:      // Mark as correct
204:      richTextBox1.SelectionFont = normalFont;
205:      richTextBox1.SelectionColor = Color.Black;
206:     }
208:     // Update to point to the character after the current word
209:     index = index + s.Length + 1;
210:    }
211:   }
212:   catch (Exception ex)
213:   {
214:    MessageBox.Show("Unable to check spelling. " + ex.Message + 
215:     "\n\nDetails:\n" + ex.ToString(), "Unable to Check Spelling", 
216:     MessageBoxButtons.OK, MessageBoxIcon.Error);
217:   }
218:  }
220:  // Called when the user clicks anywhere on the text. If the user 
221:  // clicked the right mouse button, this determines the word underneath
222:  // the mouse pointer (if any) and presents a context menu of
223:  // spelling suggestions.
224:  private void richTextBox1_MouseDown(object sender, MouseEventArgs e)
225:  {
226:   try
227:   {
228:    if (e.Button == MouseButtons.Right)
229:    {
230:     // Get the location of the mouse pointer
231:     Point point = new Point(e.X, e.Y);
233:     // Find the index of the character underneath the mouse pointer
234:     int index = richTextBox1.GetCharIndexFromPosition(point);
236:     // Length of the word underneath the mouse pointer
237:     int length = 1;
239:     // If the character under the mouse pointer isn't whitespace,
240:     // determine what the word is and display spelling suggestions
242:     if (!Char.IsWhiteSpace(richTextBox1.Text[index]))
243:     {
244:      // Going backward from the index, 
245:      // figure out where the word begins
246:      while (index > 0 && !Char.IsWhiteSpace(
247:       richTextBox1.Text[index-1])) { index--; length++; }
249:      // Going forward, figure out where the word ends, 
250:      // making sure to not include punctuation except for apostrophes
251:      // (This works for English.)
252:      while (index + length < richTextBox1.Text.Length &&
253:       !Char.IsWhiteSpace(richTextBox1.Text[index + length]) &&
254:       (!Char.IsPunctuation(richTextBox1.Text[index + length]) || 
255:       richTextBox1.Text[index + length] == Char.Parse("'"))
256:       ) length++;
258:      // Now that we've found the entire word, select it
259:      richTextBox1.SelectionStart = index;
260:      richTextBox1.SelectionLength = length;
262:      // Clear the context menu in case 
263:      // there are items on it from last time
264:      contextMenu1.MenuItems.Clear();
266:      // Enumerate over the SpellingSuggestions collection 
267:      // returned by GetSpellingSuggestions
268:      foreach (Word.SpellingSuggestion s in 
269:       GetSpellingSuggestions(richTextBox1.SelectedText))
270:      {
271:       // Add the menu item with the suggestion text and the 
272:       // Menu_Click handler
273:       contextMenu1.MenuItems.Add(s.Name, menuHandler);
274:      }
276:      // Display special text if there are no spelling suggestions
277:      if (contextMenu1.MenuItems.Count == 0)
278:      {
279:       contextMenu1.MenuItems.Add("No suggestions.");
280:      }
281:      else
282:      {
283:       // Add two more items whenever there are spelling suggestions.
284:       // Since there is no event handler, nothing will happen when 
285:       // these are clicked.
286:       contextMenu1.MenuItems.Add("-");
287:       contextMenu1.MenuItems.Add("Don't change the spelling.");
288:      }
290:      // Now that the menu is ready, show it
291:      contextMenu1.Show(richTextBox1, point);
292:     }
293:    }
294:   }
295:   catch (Exception ex)
296:   {
297:    MessageBox.Show("Unable to give spelling suggestions. " + 
298:     ex.Message + "\n\nDetails:\n" + ex.ToString(), 
299:     "Unable to Give Spelling Suggestions", MessageBoxButtons.OK, 
300:     MessageBoxIcon.Error);
301:   }
302:  }
304:  // Called when a spelling suggestion is clicked on the context menu.
305:  // Replaces the currently selected text with the text 
306:  // from the item clicked.
307:  private void Menu_Click(object sender, EventArgs e)
308:  {
309:   // The suggestion should be spelled correctly, 
310:   // so restore the font to normal
311:   richTextBox1.SelectionFont = normalFont;
312:   richTextBox1.SelectionColor = Color.Black;
314:   // Obtain the text from the MenuItem and replace the SelectedText
315:   richTextBox1.SelectedText = ((MenuItem)sender).Text;
316:  }
317: }

Line 17 declares the Application object used to communicate with Microsoft Word, and Lines 20–21 define the two different fonts that the application uses—one for correctly spelled words and one for misspelled words. Line 25 defines a Missing instance that is used for accepting the default behavior of optional parameters. It's defined as a System.Object due to C#'s requirement of exact type matching when passing by-reference parameters. The ignoreUpper variable in Line 26 tracks the user's preference about checking uppercase words, and the menuHandler delegate in Line 30 handles clicks on the context menu presented when a user right-clicks. Events and delegates are discussed in Chapter 5.

The OnLoad method in Lines 41–81 handles the initialization of Microsoft Word. If Word isn't installed, it displays a warning message and simply disables spell checking functionality rather than ending the entire program. The CheckSpelling method in Lines 113–133 returns true if the input word is spelled correctly, or false if it is misspelled (according to Word's own CheckSpelling method). The GetSpellingSuggestions method in Lines 140–161 returns a SpellingSuggestions collection returned by Word for the input string. These CheckSpelling and GetSpellingSuggestions methods wrap their corresponding Word methods simply because the original methods are cumbersome with all of the optional parameters that must be dealt with explicitly.

The button1_Click method in Lines 166–218, which is called when the user clicks the Check Spelling button, enumerates over every word inside the RichTextBox control and calls CheckSpelling to determine whether to underline each word. The richTextBox1_MouseDown method in Lines 224–302 determines if the user has right-clicked on a word. If so, it dynamically builds a context menu with the collection of spelling suggestions returned by the call to GetSpellingSuggestions in Line 269. The Menu_Click method in Lines 307–316 is the event handler associated with any misspelled words displayed on the context menu. When the user clicks a word in the menu, this method is called and Line 315 replaces the original text with the corrected text.

Microsoft Word is an out-of-process COM server, meaning that it runs in a separate process from the application that's using it. You can see this process while the example application runs by opening Windows Task Manager and looking for WINWORD.EXE.

Using Windows Task Manager, here's something you can try to see exception handling in action:

  1. Start the example application.

  2. Open Windows Task Manager by Pressing Ctrl+Alt+Delete (and, if appropriate, clicking the Task Manager button) and end the WINWORD.EXE process, as shown in Figure 3.16. The exact step depends on your version of Windows, but there should be a button marked either End Process or End Task.

  3. Although the server has been terminated, the client application is still running. Now press the Check Spelling button on the example application.

  4. Observe the dialog box that appears, which is shown in Figure 3.17. This is the result of the catch statement in the button1_Click method.

Figure 3.16 Ending the WINWORD.EXE process while the client application is still running.

Figure 3.17 Handling a comexcetion caused by a failure hresult.


This chapter discussed a bunch of topics that are essential for using COM components in .NET applications. In this chapter, you've seen how to reference COM components via Interop Assemblies generated by the type library importer. Primary Interop Assemblies were also introduced, so you should understand why they should be used whenever possible instead of generating your own Interop Assemblies.

You've also seen the main ways to create a COM object:

With metadata:

new or Type.GetType + Activator.CreateInstance

Without metadata:

Type.GetTypeFromProgID or Type.GetTypeFromCLSID + Activator.CreateInstance

You've also seen how to invoke a COM object's methods via its v-table, or by late binding. The next chapter examines Interop Assemblies in detail so you can gain a better understanding on what to expect when using any COM objects in managed code, and how it differs from using them in COM-based languages.

posted on 2005-07-31 13:19  cy163  阅读(7284)  评论(0编辑  收藏  举报