Long Paths in .NET, Part 2 of 3 [Kim Hamilton][译]

就目前来说, 我们建议那些碰到MAX_PATH问题的客户重新安排他们的目录从而使文件路径变的短一些. 这看起来像是个不负责任的逃跑吧? 不过对于用户来说却是最简单的, 原因有二: 1, 不需要额外的工具来访问文件(Explorer不能打开long path的文件, 你必须自己写一个!) 2, 能够充分使用System.IO提供的功能而不需要自己再写n多的重复代码. 然而, 如果你真的需要使用超过MAX_PATHlong path的话, 我会给你展示一些demo.

上次我们已经说到可以在Unicode版本的Win32 API中使用”\\?\”前缀就可以处理最大32k的路径了. 下面这些代码展示使用这种方法来做一些常规的文件操作.

删除一个文件

我们还是从最简单的开始吧 删除一个文件. 不要忘记Explorer不能替你删除那些long path的文件, 那你就不得不自己擦屁股了.

首先, 我们要确认一下Win32 API中的DeleteFile是不是支持long path. 从下面这段来看答案是肯定的:

In the ANSI version of this function, the name is limited to MAX_PATH characters. To extend this limit to 32,767 wide characters, call the Unicode version of the function and prepend "\\?\" to the path. For more information, see Naming a File.


那么我们先添加
PInvoke signature:

           

using System;

using System.Runtime.InteropServices;

 

[DllImport("kernel32.dll", CharSet = CharSet.Unicode)]

[return: MarshalAs(UnmanagedType.Bool)]

internal static extern bool DeleteFile(string lpFileName);


然后我们所要做的就是在文件名前面\\?\:


// This code snippet is provided under the Microsoft Permissive License.

public static void Delete(string fileName) {

    string formattedName = @"\\?\" + fileName;

    DeleteFile(formattedName);

}

           
对于一些简单的文件操作, 比如删除, 移动和重命名,像上面这样直接API就搞定了. 另外有些复杂的情况, 比如写一个文件, 你可以混合调用Win32 APISystem.IO.

写入和读取一个文件

 

首先你要调用Win32 CreateFile方法来创建或打开一个文件. CreateFile会返回一个文件句柄, 你可以把它传给System.IO.FileStream的构造函数. 剩下的就由FileStream搞定了.

           

// This code snippet is provided under the Microsoft Permissive License.

using System;

using System.IO;

using System.Runtime.InteropServices;

using Microsoft.Win32.SafeHandles;

 

[DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]

internal static extern SafeFileHandle CreateFile(

    string lpFileName,

    EFileAccess dwDesiredAccess,

    EFileShare dwShareMode,

    IntPtr lpSecurityAttributes,

    ECreationDisposition dwCreationDisposition,

    EFileAttributes dwFlagsAndAttributes,

    IntPtr hTemplateFile);

 

public static void TestCreateAndWrite(string fileName) {

 

    string formattedName = @"\\?\" + fileName;

    // Create a file with generic write access

    SafeFileHandle fileHandle = CreateFile(formattedName,

        EFileAccess.GenericWrite, EFileShare.None, IntPtr.Zero,

        ECreationDisposition.CreateAlways, 0, IntPtr.Zero);

 

    // Check for errors

    int lastWin32Error = Marshal.GetLastWin32Error();

    if (fileHandle.IsInvalid) {

        throw new System.ComponentModel.Win32Exception(lastWin32Error);

    }

 

    // Pass the file handle to FileStream. FileStream will close the

    // handle

    using (FileStream fs = new FileStream(fileHandle,

                                    FileAccess.Write)) {

        fs.WriteByte(80);

        fs.WriteByte(81);

        fs.WriteByte(83);

        fs.WriteByte(84);

    }

}


在这个例子里我们往文件里面写了一些数据. 但是一旦你构造了FileStream, 你就可以像往常一样使用它了. 比如把它包进一个BinaryWriter等等.

如果你只是想打开而不是新建一个文件, 那么就把参数dwCreationDispositionCreateAlways改成OpenExisting. 如果你想读取而不是写入一个文件, 那么就把参数dwDesiredAccessGenericWrite改成GenericRead.

本文结束的部分有这么枚举的定义.

查找文件和目录

 

到目前为止我们方案看起来不错. 但是设想一下你要找到某个特定文件夹下面的子文件夹和文件...ah...看起来我们像是在重写.NET类库. J            

// This code snippet is provided under the Microsoft Permissive License.

using System;

using System.Collections.Generic;

using System.IO;

using System.Runtime.InteropServices;

using Microsoft.Win32.SafeHandles;

 

[DllImport("kernel32.dll", CharSet = CharSet.Unicode)]

internal static extern IntPtr FindFirstFile(string lpFileName, out

                                WIN32_FIND_DATA lpFindFileData);

 

[DllImport("kernel32.dll", CharSet = CharSet.Unicode)]

internal static extern bool FindNextFile(IntPtr hFindFile, out

                                WIN32_FIND_DATA lpFindFileData);

 

[DllImport("kernel32.dll", SetLastError = true)]

[return: MarshalAs(UnmanagedType.Bool)]

internal static extern bool FindClose(IntPtr hFindFile);

 

// Assume dirName passed in is already prefixed with \\?\

public static List<string> FindFilesAndDirs(string dirName) {

 

    List<string> results = new List<string>();

    WIN32_FIND_DATA findData;

    IntPtr findHandle = FindFirstFile(dirName + @"\*", out findData);

 

    if (findHandle != INVALID_HANDLE_VALUE) {

        bool found;

        do {

            string currentFileName = findData.cFileName;

 

            // if this is a directory, find its contents

            if (((int)findData.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY) != 0) {

                if (currentFileName != "." && currentFileName != "..")

                {

                List<string> childResults = FindFilesAndDirs(Path.Combine(dirName, currentFileName));

                // add children and self to results

                results.AddRange(childResults);

                results.Add(Path.Combine(dirName, currentFileName));

                }

            }

 

            // it's a file; add it to the results

            else {

                results.Add(Path.Combine(dirName, currentFileName));

            }

 

            // find next

            found = FindNextFile(findHandle, out findData);

        }

        while (found);

    }

 

    // close the find handle

    FindClose(findHandle);

    return results;

}

   

相关资源:

Constants, StructsEnums:

internal static IntPtr INVALID_HANDLE_VALUE = new IntPtr(-1);

internal static int FILE_ATTRIBUTE_DIRECTORY = 0x00000010;

internal const int MAX_PATH = 260;

 

[StructLayout(LayoutKind.Sequential)]

internal struct FILETIME {

    internal uint dwLowDateTime;

    internal uint dwHighDateTime;

};

 

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]

internal struct WIN32_FIND_DATA {

    internal FileAttributes dwFileAttributes;

    internal FILETIME ftCreationTime;

    internal FILETIME ftLastAccessTime;

    internal FILETIME ftLastWriteTime;

    internal int nFileSizeHigh;

    internal int nFileSizeLow;

    internal int dwReserved0;

    internal int dwReserved1;

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAX_PATH)]

    internal string cFileName;

    // not using this

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]

    internal string cAlternate;

}

 

[Flags]

public enum EFileAccess : uint {

    GenericRead = 0x80000000,

    GenericWrite = 0x40000000,

    GenericExecute = 0x20000000,

    GenericAll = 0x10000000,

}

 

[Flags]

public enum EFileShare : uint {

    None = 0x00000000,

    Read = 0x00000001,

    Write = 0x00000002,

    Delete = 0x00000004,

}

 

public enum ECreationDisposition : uint {

    New = 1,

    CreateAlways = 2,

    OpenExisting = 3,

    OpenAlways = 4,

    TruncateExisting = 5,

}

 

[Flags]

public enum EFileAttributes : uint {

    Readonly = 0x00000001,

    Hidden = 0x00000002,

    System = 0x00000004,

    Directory = 0x00000010,

    Archive = 0x00000020,

    Device = 0x00000040,

    Normal = 0x00000080,

    Temporary = 0x00000100,

    SparseFile = 0x00000200,

    ReparsePoint = 0x00000400,

    Compressed = 0x00000800,

    Offline = 0x00001000,

    NotContentIndexed = 0x00002000,

    Encrypted = 0x00004000,

    Write_Through = 0x80000000,

    Overlapped = 0x40000000,

    NoBuffering = 0x20000000,

    RandomAccess = 0x10000000,

    SequentialScan = 0x08000000,

    DeleteOnClose = 0x04000000,

    BackupSemantics = 0x02000000,

    PosixSemantics = 0x01000000,

    OpenReparsePoint = 0x00200000,

    OpenNoRecall = 0x00100000,

    FirstPipeInstance = 0x00080000

}

 

[StructLayout(LayoutKind.Sequential)]

public struct SECURITY_ATTRIBUTES {

    public int nLength;

    public IntPtr lpSecurityDescriptor;

    public int bInheritHandle;

}

posted @ 2007-04-26 18:12  光阴四溅  阅读(753)  评论(0编辑  收藏  举报