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_PATH的long 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.
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加\\?\:
// 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 API和System.IO.
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dwCreationDisposition从CreateAlways改成OpenExisting. 如果你想读取而不是写入一个文件, 那么就把参数dwDesiredAccess从GenericWrite改成GenericRead.
本文结束的部分有这么枚举的定义.
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;
}
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;
}
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