我有一个代码块，需要使用 UNC 路径从 NAS 服务器打开和读取大量小文本文件。 此代码是最初用 C++ 编写的模块的一部分，但现在正在转换为 C#。 C# 版本明显较慢。 我确定打开文件的调用几乎占了所有的性能差异。 使用WireShark，我发现这是因为System.IO.File.Open调用发出的SMB网络请求比类似的C++代码多得多。
C++代码进行以下调用：
FILE *f = _wfsopen(fileName, L"r", _SH_DENYWR);
这将生成以下 SMB 请求序列：
NT Create AndX Request, FID: 0x0004, Path: \a\i\a\q\~141106162638847.nmd
NT Create AndX Response, FID: 0x0004
Trans2 Request, QUERY_FILE_INFO, FID: 0x0004, Query File Basic Info
Trans2 Response, FID: 0x0004, QUERY_FILE_INFO
Read AndX Request, FID: 0x0004, 1327 bytes at offset 0
Read AndX Response, FID: 0x0004, 1327 bytes
Close Request, FID: 0x0004
Close Response, FID: 0x0004
NT Create AndX Request, FID: 0x0005, Path: \a\i\a\q\~141106162638847.nmd
NT Create AndX Response, FID: 0x0005
C# 代码进行此调用：
FileStream f = File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.Read);
这将生成以下 SMB 请求序列：
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a\i\a\q\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \a\i\a\q\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a\i\a\q\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path:
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path:
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \a
Trans2 Response, FIND_FIRST2, Files: a
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \a\i
Trans2 Response, FIND_FIRST2, Files: i
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a\i
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \a\i
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \a\i\a
Trans2 Response, FIND_FIRST2, Files: a
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a\i\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \a\i\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \a\i\a\q
Trans2 Response, FIND_FIRST2, Files: q
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a\i\a\q\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \a\i\a\q\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a\i\a\q\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path:
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path:
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \a
Trans2 Response, FIND_FIRST2, Files: a
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \a\i
Trans2 Response, FIND_FIRST2, Files: i
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a\i
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \a\i
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \a\i\a
Trans2 Response, FIND_FIRST2, Files: a
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \a\i\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \a\i\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \a\i\a\q
Trans2 Response, FIND_FIRST2, Files: q
Close Request, FID: 0x000f
Close Response
NT Create AndX Request, FID: 0x0018, Path: \a\i\a\q\~141106162638847.nmd
NT Create AndX Response, FID: 0x0018
Trans2 Request, QUERY_FILE_INFO, FID: 0x0018, Query File Basic Info
Trans2 Response, FID: 0x0018, QUERY_FILE_INFO
Read AndX Request, FID: 0x0018, 1327 bytes at offset 0
Read AndX Response, FID: 0x0018, 1327 bytes
Close Request, FID: 0x0018
Close Response, FID: 0x0018
NT Create AndX Request, FID: 0x0019, Path: \a\i\a\q\~141106162638847.nmd
NT Create AndX Response, FID: 0x0019
为什么System.IO.File.Open会发出所有这些额外的SMB请求？ 有没有办法更改此代码以避免所有这些额外的请求？
简而言之，File.Open 调用new FileStream()，new FileStream() 会做很多调用：
-
规范化路径。
String filePath = Path.NormalizePath(path, true, maxPath); // fullCheck: true
导致此代码：
1.a：获取完整路径：
if (fullCheck) { ...
result = newBuffer.GetFullPathName();
GetFullPathName（） 调用Win32Native.GetFullPathName一两次（取决于结果路径的长度）。
1.b. 试图扩大短途。您的路径包含~字符，因此它看起来像是扩展路径的候选路径：
if (mightBeShortFileName) {
bool r = newBuffer.TryExpandShortFileName();
因此，调用了 Win32Native.GetLongPathName（）。
FileIoPermission.Demand（） （仅适用于非受信任）：
// All demands in full trust domains are no-ops, so skip if (!CodeAccessSecurityEngine.QuickCheckForAllDemands()) { ... new FileIOPermission(secAccess, control, new String[] { filePath }, false, false).Demand();打开文件流（软盘反击;））：
// Don't pop up a dialog for reading from an emtpy floppy drive int oldMode = Win32Native.SetErrorMode(Win32Native.SEM_FAILCRITICALERRORS); try { ... _handle = Win32Native.SafeCreateFile(tempPath, fAccess, share, secAttrs, mode, flagsAndAttributes, IntPtr.Zero);Win32Native.GetFileType（）
并非所有这些都会导致 smb 请求，但有些会导致。我尝试通过使用源逐步调试（这是启用 .net 源调试的手册）并在每个步骤后检查日志来重现聊天请求。Resuts更类似于您的首次上市。如果你真的有兴趣找到真正的问题，你必须自己做。
UPD 请注意，我已经检查了当前 （.net 4.5.2） 行为。自 2.0 以来，它被多次更改（例如 FileIOPermission.Demand()最初也是为完全受信任的代码调用的），所以这取决于：）
对于为什么 .NET 实现如此健谈，我真的没有具体的答案，但是此行为是由于System.IO.FileStream的实现，因为File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.Read);所做的只是将参数传递给 FileStream 构造函数。
public static FileStream Open(string path, FileMode mode, FileAccess access, FileShare share)
{
return new FileStream(path, mode, access, share);
}
更改FileStream的行为意味着您基本上必须重新实现FileStream类，这将需要大量的努力。
另一个更简单的替代方法是创建一个本机包装器，该包装器调用您给出的C++代码。然后从 C# 代码调用本机包装器。