如何克服PathTooLongException？

Question

因此，我正在编写一个程序，它将扫描计算机上的重复文件，因为我看到的程序非常慢，而且/或内存占优势，但是当我尝试整个驱动器时，我遇到了一个PathTooLongException。在阅读了PathTooLongException in C# code之后，我对以下两个问题感到好奇。

如果每次更改级别时切换当前目录，

1. 会损害我的性能吗？
2. 是否有更好的方法来获取所有文件的目录结构(可能通过调用tree.exe之类的内容，然后解析它)？

Answer 1

看这个图书馆！

.NET Base Class Libraries : Long Path

Answer 2

或者自己动手，

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 IEnumerable<string> EnumerateEntries(string directory)
{ 
    WIN32_FIND_DATA findData;
    IntPtr findHandle = FindFirstFile(dirName + @"\*", out findData);

    try
    {
        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 != "..")
                    {
                        foreach(var child in FindFilesAndDirs(
                                Path.Combine(dirName, currentFileName))
                        {
                            yield return child;
                        }
                    }
                }

                yield return Path.Combine(dirName, currentFileName);

                // find next
                found = FindNextFile(findHandle, out findData);
            }
            while (found);
        }

    }
    finally
    {
        // close the find handle
        FindClose(findHandle);
    }
}

我还没有验证这段代码，而且显然并不是所有的类型都定义好了，但是它应该会为我们指明正确的方向。

Answer 3

纯C#，需要优化，但可以在不使用外部库或p/调用的情况下给人们一个优势。

public static class DirectoryEx
{
    static char driveLetter;
    static string longPath;
    static List<string> directories;

    static DirectoryEx()
    {
        longPath = String.Empty;
    }

    private static char GetAvailableDrive()
    {
        var all = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray().Reverse();
        var occupied = DriveInfo.GetDrives()
            .OrderByDescending(d => d.Name)
            .Select(d => (char)d.Name.ToUpper().First());

        var free = all.Except(occupied).First();

        return free;
    }

    public static List<string> GetDirectories(string path)
    {
        directories = new List<string>();

        // recursive call
        FindDirectories(path);

        return directories;
    }

    static void FindDirectories(string path)
    {
        try
        {
            foreach (var directory in Directory.GetDirectories(path))
            {
                var di = new DirectoryInfo(directory);

                if(!String.IsNullOrEmpty(longPath))
                    directories.Add(di.FullName.Replace(driveLetter + ":\\", longPath + "\\"));
                else
                    directories.Add(di.FullName);

                FindDirectories(di.FullName);
            }
        }
        catch (UnauthorizedAccessException uaex) { Debug.WriteLine(uaex.Message); }
        catch (PathTooLongException ptlex)
        {
            Debug.WriteLine(ptlex.Message);

            longPath = path;

            Task t = new Task(new Action(() =>
            {
                CreateVirtualDrive(longPath);
                FindDirectories(driveLetter + ":\\");
                DeleteVirtualDrive();

                longPath = String.Empty;
            }));

            if (!String.IsNullOrEmpty(longPath))
                t.RunSynchronously();
        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex.Message); 
        }
    }

    static void CreateVirtualDrive(string path)
    {
        driveLetter = GetAvailableDrive();

        Process.Start(new ProcessStartInfo() {
            FileName = "cmd.exe",
            WindowStyle = ProcessWindowStyle.Hidden,
            Arguments = String.Format("/c subst {0}: {1}", driveLetter.ToString(), path)
        });

        while (!DriveInfo.GetDrives().Select(d => d.Name.ToUpper().First()).Contains(driveLetter))
        {
            System.Threading.Thread.Sleep(1);
        }
    }

    static void DeleteVirtualDrive()
    {
        Process.Start(new ProcessStartInfo()
        {
            FileName = "cmd.exe",
            WindowStyle = ProcessWindowStyle.Hidden,
            Arguments = String.Format("/c subst {0}: /D", driveLetter.ToString())
        });

        while (DriveInfo.GetDrives().Select(d => d.Name.ToUpper().First()).Contains(driveLetter))
        {
            System.Threading.Thread.Sleep(1);
        }
    }
}