我能否获取内存映射文件的路径？（.NET 4.0）

Question

我希望一款非.NET应用程序能够访问一个内存映射文件，但是这个应用程序不知道内存映射文件的存在，所以我需要文件路径。这可能吗？

- Jader Dias

---

好问题...不幸的是，答案似乎是否定的。 - Noldorin

Answer 1

他们有一些示例在这里。 编辑 我认为这个会提供答案。基本上，似乎需要某种内存指针来进行内存映射文件，而不是文件系统路径。

- Kenan E. K.

Answer 2

您可以使用 GetMappedFileName 函数获取映射文件的路径，当它被映射时。当然，这需要内存映射文件实际上由物理文件支持，但问题使情况变得模糊不清。一些第三方库是否将 MemoryMappedFile、MemoryMappedViewAccessor 或 MemoryMappedViewStream 交给您了，但您不知道它是否由物理文件支持？

下面是一个示例，展示如何从 MemoryMappedFile 中获取文件名：

using Microsoft.Win32.SafeHandles;
using System;
using System.ComponentModel;
using System.Diagnostics;
using System.IO.MemoryMappedFiles;
using System.Runtime.InteropServices;
using System.Text;

namespace MMFilePathTest
{
    static class Program
    {
        private static MemoryMappedFile GetMappedPhysicalFile()
        {
            return MemoryMappedFile.CreateFromFile("test.bin", System.IO.FileMode.Create, null, 4096);
        }

        private static MemoryMappedFile GetMappedAnonymousMemory()
        {
            /* The documentation errounously claims that mapName must not be null. Actually anonymous
             * mappings are quite a normal thing on Windows, and is actually both safer and more secure
             * if you don't have a need for a name for them anyways.
             * (Reported as https://github.com/dotnet/docs/issues/5404)
             * Using a name here gives the exact same results (assuming the name isn't already in use). */
            return MemoryMappedFile.CreateNew(null, 4096);
        }

        /* This can be changed to kernel32.dll / K32GetMappedFileNameW if compatibility with Windows Server 2008 and
         * earlier is not needed, but it is not clear what the gain of doing so is, see the remarks about
         * PSAPI_VERSION at https://msdn.microsoft.com/en-us/library/windows/desktop/ms683195(v=vs.85).aspx */
        [DllImport("Psapi.dll", EntryPoint = "GetMappedFileNameW", CharSet = CharSet.Unicode, ExactSpelling = true, SetLastError = true)]
        private static extern int GetMappedFileName(
          SafeProcessHandle hProcess,
          SafeMemoryMappedViewHandle lpv,
          [Out] StringBuilder lpFilename,
          int nSize
        );

        /* Note that the SafeMemoryMappedViewHandle property of SafeMemoryMappedViewAccess and SafeMemoryMappedViewStream
         * is actually the address where the file is mapped */
        private static string GetPathWithGetMappedFileName(SafeMemoryMappedViewHandle memoryMappedViewHandle)
        {
            // The maximum path length in the NT kernel is 32,767 - memory is cheap nowadays so its not a problem 
            // to just allocate the maximum size of 32KB right away.
            StringBuilder filename = new StringBuilder(short.MaxValue);
            int len;
            len = GetMappedFileName(Process.GetCurrentProcess().SafeHandle, memoryMappedViewHandle, filename, short.MaxValue);
            if (len == 0)
                throw new Win32Exception(Marshal.GetLastWin32Error());
            filename.Length = len;
            return filename.ToString();
        }

        private static void PrintFileName(MemoryMappedFile memoryMappedFile)
        {
            try
            {
                using (memoryMappedFile)
                using (MemoryMappedViewAccessor va = memoryMappedFile.CreateViewAccessor())
                {
                    string filename = GetPathWithGetMappedFileName(va.SafeMemoryMappedViewHandle);
                    Console.WriteLine(filename);
                }
            }
            catch (Win32Exception e)
            {
                Console.WriteLine("Error: 0x{0:X08}: {1}", e.NativeErrorCode, e.Message);
            }
        }

        static void Main(string[] args)
        {
            PrintFileName(GetMappedPhysicalFile());
            PrintFileName(GetMappedAnonymousMemory());
        }
    }
}

当我运行这段代码时，输出结果为：

\Device\HarddiskVolume5\Users\poizan\Documents\Visual Studio 2017\Projects\MMFilePathTest\MMFilePathTest\bin\Debug\test.bin
Error: 0x000003EE: The volume for a file has been externally altered so that the opened file is no longer valid

请注意，路径以本机NT路径格式返回。如果您需要将其转换为dos/win32格式，请参阅此问题：如何将本机（NT）路径名转换为Win32路径名？ 当没有与之关联的文件时出现错误时，错误消息有点奇怪，但错误代码意味着ERROR_FILE_INVALID，这是有道理的，因为没有文件。

- poizan42