c# - 为什么 Path.GetDirectoryName 函数必须依赖于 260 个字符的限制

Question

public static void Main()
{
    // testing file name
    var fileName =
        "\\\\server7\\EmailAttachments\\myfolder\\abc\\2012\\1126\\e85c6b82-edc5-4ce1-9ad0-8025b92d0105-o.dom=38c55279fe168c290fb2b06a312b5d88&b=6f54a59ce903eeaff197f82937de4012.dom=38c55279fe168c290fb2b06a312b5d88&b=6f54a59ce903eeaff197f82937de4012=6f54a59ce903eeaff197f82937de4012.dom=38c55279fe168c290fb2b06a312b5d88&b=6f54a59ce903eeaff197f82937de4012";

    var directory = fileName.GetDirectory();
}

   public static string GetDirectory(this string fullyQualifiedFileName)
     {
         return Path.GetDirectoryName(fullyQualifiedFileName); // throwing exception here
     }

低于异常

System.IO.PathTooLongException occurred HResult=-2147024690
Message=The specified path, file name, or both are too long. The fully qualified file name must be less than 260 characters, and the directory name must be less than 248 characters. Source=mscorlib
StackTrace: at System.IO.Path.NormalizePath(String path, Boolean fullCheck, Int32 maxPathLength, Boolean expandShortPaths) at System.IO.Path.GetDirectoryName(String path) at Sameer.FilePathExtension.GetDirectory(String fullyQualifiedFileName) in f:\Practice Projects\Sameer\Sameer\FilePathExtension.cs:line 137 InnerException:

我想知道为什么 GetDirectoryName 必须依赖于路径或文件名字符限制。

Answer 1

如 msdn 网站中所述

"Full paths must not exceed 260 characters to maintain compatibility with Windows operating systems"

http://msdn.microsoft.com/en-us/library/system.io.pathtoolongexception(v=vs.100).aspx

您可以在此处找到其背后的原因，如以下链接中所述。

Another concern is inconsistent behavior that would result by exposing long path support. Long paths with the \?\ prefix can be used in most of the file-related Windows APIs, but not all Windows APIs. For example, LoadLibrary, which maps a module into the address of the calling process, fails if the file name is longer than MAX_PATH. So this means MoveFile will let you move a DLL to a location such that its path is longer than 260 characters, but when you try to load the DLL, it would fail. There are similar examples throughout the Windows APIs; some workarounds exist, but they are on a case-by-case basis.

Another factor, which is considered more of a pain factor, is compatibility with other Windows-based applications and the Windows shell itself, which only work with paths shorter than MAX_PATH (note that the Vista shell attempts to soften this limit, briefly described below). This means that if .NET supports long paths, then we’d let you create files that you couldn’t access via Explorer or the command prompt.

That said, we realize that enforcing the 260 character limit isn’t a reasonable long-term solution. Our customers don’t run into this problem very often, but when they do, they’re extremely inconvenienced. A possible workaround is P/Invoking to the Windows APIs and using the \?\ prefix, but it would be a huge amount of code to duplicate the functionality in System.IO. So to work around the problem, customers often resort to an overhaul of directory organization, artificially shortening directory names (and updating every referencing location).

http://blogs.msdn.com/b/bclteam/archive/2007/02/13/long-paths-in-net-part-1-of-3-kim-hamilton.aspx