《CLR Via C# 第3版》笔记之（六） - IL中的call和callvirt
C#中调用一个函数时生成的IL代码有两种形式，分别为call 和 callvirt。
主要内容
- call和callvirt的区别
- call和callvirt的例子
1. call和callvirt的区别
call的callvirt的区别主要有两点：
1）call可以调用静态方法，实例方法和虚方法
callvirt只能调用实例方法和虚方法，不能调用静态方法
2）call一般是以非虚的方式来调用函数的
callvirt是以已多态的方式来调用函数的
2. call和callvirt的例子
示例代码如下：
using System;
namespace Test6
{
public class CLRviaCSharp_6
{
static void Main(string[] args)
{
BaseClass.SShow();
BaseClass b = new BaseClass();
BaseClass s = new SubClass();
b.VShow();
s.VShow();
Console.ReadKey(true);
}
}
public class BaseClass
{
public static void SShow()
{
Console.WriteLine("Base class static method: SShow()!");
}
public virtual void VShow()
{
Console.WriteLine("Base class virtual method: VShow()!");
}
}
public class SubClass : BaseClass
{
public override void VShow()
{
Console.WriteLine("Sub class virtual method: VShow()!");
}
}
}
程序执行结果为：
利用ildasm.exe将上面代码生成的exe文件反编译为IL代码。
命令：ildasm .\Test6.exe /output:Test6.il
IL代码如下：
// Microsoft (R) .NET Framework IL Disassembler. Version 3.5.30729.1
// Copyright (c) Microsoft Corporation. All rights reserved.
// Metadata version: v4.0.30319
.assembly extern mscorlib
{
.publickeytoken = (B7 7A 5C 56 19 34 E0 89 ) // .z\V.4..
.ver 4:0:0:0
}
.assembly Test6
{
.custom instance void [mscorlib]System.Reflection.AssemblyTitleAttribute::.ctor(string) = ( 01 00 0C 63 6E 62 6C 6F 67 5F 62 6F 77 65 6E 00 // ...cnblog_bowen.
00 )
.custom instance void [mscorlib]System.Reflection.AssemblyDescriptionAttribute::.ctor(string) = ( 01 00 00 00 00 )
.custom instance void [mscorlib]System.Reflection.AssemblyConfigurationAttribute::.ctor(string) = ( 01 00 00 00 00 )
.custom instance void [mscorlib]System.Reflection.AssemblyCompanyAttribute::.ctor(string) = ( 01 00 18 4C 65 6E 6F 76 6F 20 28 42 65 69 6A 69 // ...Lenovo (Beiji
6E 67 29 20 4C 69 6D 69 74 65 64 00 00 ) // ng) Limited..
.custom instance void [mscorlib]System.Reflection.AssemblyProductAttribute::.ctor(string) = ( 01 00 0C 63 6E 62 6C 6F 67 5F 62 6F 77 65 6E 00 // ...cnblog_bowen.
00 )
.custom instance void [mscorlib]System.Reflection.AssemblyCopyrightAttribute::.ctor(string) = ( 01 00 2A 43 6F 70 79 72 69 67 68 74 20 C2 A9 20 // ..*Copyright ..
4C 65 6E 6F 76 6F 20 28 42 65 69 6A 69 6E 67 29 // Lenovo (Beijing)
20 4C 69 6D 69 74 65 64 20 32 30 31 30 00 00 ) // Limited 2010..
.custom instance void [mscorlib]System.Reflection.AssemblyTrademarkAttribute::.ctor(string) = ( 01 00 00 00 00 )
.custom instance void [mscorlib]System.Runtime.InteropServices.ComVisibleAttribute::.ctor(bool) = ( 01 00 00 00 00 )
.custom instance void [mscorlib]System.Runtime.InteropServices.GuidAttribute::.ctor(string) = ( 01 00 24 61 64 61 62 34 66 63 33 2D 33 33 64 35 // ..$adab4fc3-33d5
2D 34 62 64 30 2D 39 61 32 61 2D 39 35 35 61 65 // -4bd0-9a2a-955ae
66 38 35 33 31 62 37 00 00 ) // f8531b7..
.custom instance void [mscorlib]System.Reflection.AssemblyFileVersionAttribute::.ctor(string) = ( 01 00 07 31 2E 30 2E 30 2E 30 00 00 ) // ...1.0.0.0..
.custom instance void [mscorlib]System.Runtime.Versioning.TargetFrameworkAttribute::.ctor(string) = ( 01 00 29 2E 4E 45 54 46 72 61 6D 65 77 6F 72 6B // ..).NETFramework
2C 56 65 72 73 69 6F 6E 3D 76 34 2E 30 2C 50 72 // ,Version=v4.0,Pr
6F 66 69 6C 65 3D 43 6C 69 65 6E 74 01 00 54 0E // ofile=Client..T.
14 46 72 61 6D 65 77 6F 72 6B 44 69 73 70 6C 61 // .FrameworkDispla
79 4E 61 6D 65 1F 2E 4E 45 54 20 46 72 61 6D 65 // yName..NET Frame
77 6F 72 6B 20 34 20 43 6C 69 65 6E 74 20 50 72 // work 4 Client Pr
6F 66 69 6C 65 ) // ofile
// --- The following custom attribute is added automatically, do not uncomment -------
// .custom instance void [mscorlib]System.Diagnostics.DebuggableAttribute::.ctor(valuetype [mscorlib]System.Diagnostics.DebuggableAttribute/DebuggingModes) = ( 01 00 07 01 00 00 00 00 )
.custom instance void [mscorlib]System.Runtime.CompilerServices.CompilationRelaxationsAttribute::.ctor(int32) = ( 01 00 08 00 00 00 00 00 )
.custom instance void [mscorlib]System.Runtime.CompilerServices.RuntimeCompatibilityAttribute::.ctor() = ( 01 00 01 00 54 02 16 57 72 61 70 4E 6F 6E 45 78 // ....T..WrapNonEx
63 65 70 74 69 6F 6E 54 68 72 6F 77 73 01 ) // ceptionThrows.
.hash algorithm 0x00008004
.ver 1:0:0:0
}
.module Test6.exe
// MVID: {7020C5F0-2D89-4214-AD2C-E6BFBEC57CA1}
.imagebase 0x00400000
.file alignment 0x00000200
.stackreserve 0x00100000
.subsystem 0x0003 // WINDOWS_CUI
.corflags 0x00000003 // ILONLY 32BITREQUIRED
// Image base: 0x030F0000
// =============== CLASS MEMBERS DECLARATION ===================
.class public auto ansi beforefieldinit Test6.CLRviaCSharp_6
extends [mscorlib]System.Object
{
.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// Code size 41 (0x29)
.maxstack 1
.locals init ([0] class Test6.BaseClass b,
[1] class Test6.BaseClass s)
IL_0000: nop
IL_0001: call void Test6.BaseClass::SShow()
IL_0006: nop
IL_0007: newobj instance void Test6.BaseClass::.ctor()
IL_000c: stloc.0
IL_000d: newobj instance void Test6.SubClass::.ctor()
IL_0012: stloc.1
IL_0013: ldloc.0
IL_0014: callvirt instance void Test6.BaseClass::VShow()
IL_0019: nop
IL_001a: ldloc.1
IL_001b: callvirt instance void Test6.BaseClass::VShow()
IL_0020: nop
IL_0021: ldc.i4.1
IL_0022: call valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey(bool)
IL_0027: pop
IL_0028: ret
} // end of method CLRviaCSharp_6::Main
.method public hidebysig specialname rtspecialname
instance void .ctor() cil managed
{
// Code size 7 (0x7)
.maxstack 8
IL_0000: ldarg.0
IL_0001: call instance void [mscorlib]System.Object::.ctor()
IL_0006: ret
} // end of method CLRviaCSharp_6::.ctor
} // end of class Test6.CLRviaCSharp_6
.class public auto ansi beforefieldinit Test6.BaseClass
extends [mscorlib]System.Object
{
.method public hidebysig static void SShow() cil managed
{
// Code size 13 (0xd)
.maxstack 8
IL_0000: nop
IL_0001: ldstr "Base class static method: SShow()!"
IL_0006: call void [mscorlib]System.Console::WriteLine(string)
IL_000b: nop
IL_000c: ret
} // end of method BaseClass::SShow
.method public hidebysig newslot virtual
instance void VShow() cil managed
{
// Code size 13 (0xd)
.maxstack 8
IL_0000: nop
IL_0001: ldstr "Base class virtual method: VShow()!"
IL_0006: call void [mscorlib]System.Console::WriteLine(string)
IL_000b: nop
IL_000c: ret
} // end of method BaseClass::VShow
.method public hidebysig specialname rtspecialname
instance void .ctor() cil managed
{
// Code size 7 (0x7)
.maxstack 8
IL_0000: ldarg.0
IL_0001: call instance void [mscorlib]System.Object::.ctor()
IL_0006: ret
} // end of method BaseClass::.ctor
} // end of class Test6.BaseClass
.class public auto ansi beforefieldinit Test6.SubClass
extends Test6.BaseClass
{
.method public hidebysig virtual instance void
VShow() cil managed
{
// Code size 13 (0xd)
.maxstack 8
IL_0000: nop
IL_0001: ldstr "Sub class virtual method: VShow()!"
IL_0006: call void [mscorlib]System.Console::WriteLine(string)
IL_000b: nop
IL_000c: ret
} // end of method SubClass::VShow
.method public hidebysig specialname rtspecialname
instance void .ctor() cil managed
{
// Code size 7 (0x7)
.maxstack 8
IL_0000: ldarg.0
IL_0001: call instance void Test6.BaseClass::.ctor()
IL_0006: ret
} // end of method SubClass::.ctor
} // end of class Test6.SubClass
// =============================================================
// *********** DISASSEMBLY COMPLETE ***********************
// WARNING: Created Win32 resource file Test6.res
其中Main函数从63行开始。
第71行的静态方法由call来调用的，78行和81行的虚方法则是由callvirt来调用的。
为了验证call使用非虚的方式来调用方法的，我们将生成的IL文件Test6.il中81行的callvirt改为call。
修改Test6.il后保存，然后用ilasm.exe来编译此IL文件为新的exe文件。
命令：ilasm .\Test6.il /res:Test6.res /output:Test6_new.exe
然后执行Test6_new.exe，发现最后一步也是调用基类的方法，无法表现出多态性。
其实我们在写C#代码时并不用关心call和callvirt，c#编译器会帮助我们选择合适的调用方法。
只是在分析IL时经常会遇到callvirt，这里记录下来方便以后查阅。
