这段代码中的memPtr是什么？

Question

我看过这段代码，但是我发现有些混乱，所以plzz建议 这段代码中的memPtr是什么？当我运行该代码时发生错误memPtr不存在于当前上下文中。

public class WinAPI 
{ 
    [DllImport("ntdll.dll", SetLastError = true)] 
    public static extern IntPtr NtQueryInformationFile(IntPtr fileHandle, ref IO_STATUS_BLOCK IoStatusBlock, IntPtr pInfoBlock, uint length, FILE_INFORMATION_CLASS fileInformation); 

    public struct IO_STATUS_BLOCK 
    { 
     uint status; 
     ulong information; 
    } 
    public struct _FILE_INTERNAL_INFORMATION { 
     public ulong IndexNumber; 
    } 

    // Abbreviated, there are more values than shown 
    public enum FILE_INFORMATION_CLASS 
    { 
     FileDirectoryInformation = 1,  // 1 
     FileFullDirectoryInformation,  // 2 
     FileBothDirectoryInformation,  // 3 
     FileBasicInformation,   // 4 
     FileStandardInformation,  // 5 
     FileInternalInformation  // 6 
    } 

    [DllImport("kernel32.dll", SetLastError = true)] 
    public static extern bool GetFileInformationByHandle(IntPtr hFile,out BY_HANDLE_FILE_INFORMATION lpFileInformation); 

    public struct BY_HANDLE_FILE_INFORMATION 
    { 
     public uint FileAttributes; 
     public FILETIME CreationTime; 
     public FILETIME LastAccessTime; 
     public FILETIME LastWriteTime; 
     public uint VolumeSerialNumber; 
     public uint FileSizeHigh; 
     public uint FileSizeLow; 
     public uint NumberOfLinks; 
     public uint FileIndexHigh; 
     public uint FileIndexLow; 
    } 
} 

public class Test 
{ 
    public ulong ApproachA() 
    { 
      WinAPI.IO_STATUS_BLOCK iostatus=new WinAPI.IO_STATUS_BLOCK(); 

      WinAPI._FILE_INTERNAL_INFORMATION objectIDInfo = new WinAPI._FILE_INTERNAL_INFORMATION(); 

      int structSize = Marshal.SizeOf(objectIDInfo); 

      FileInfo fi=new FileInfo(@"C:\Temp\testfile.txt"); 
      FileStream fs=fi.Open(FileMode.Open,FileAccess.Read,FileShare.ReadWrite); 

      IntPtr res=WinAPI.NtQueryInformationFile(fs.Handle, ref iostatus, memPtr, (uint)structSize, WinAPI.FILE_INFORMATION_CLASS.FileInternalInformation); 

      objectIDInfo = (WinAPI._FILE_INTERNAL_INFORMATION)Marshal.PtrToStructure(memPtr, typeof(WinAPI._FILE_INTERNAL_INFORMATION)); 

      fs.Close(); 

      Marshal.FreeHGlobal(memPtr);  

      return objectIDInfo.IndexNumber; 

    } 

    public ulong ApproachB() 
    { 
      WinAPI.BY_HANDLE_FILE_INFORMATION objectFileInfo=new WinAPI.BY_HANDLE_FILE_INFORMATION(); 

      FileInfo fi=new FileInfo(@"C:\Temp\testfile.txt"); 
      FileStream fs=fi.Open(FileMode.Open,FileAccess.Read,FileShare.ReadWrite); 

      WinAPI.GetFileInformationByHandle(fs.Handle, out objectFileInfo); 

      fs.Close(); 

      ulong fileIndex = ((ulong)objectFileInfo.FileIndexHigh << 32) + (ulong)objectFileInfo.FileIndexLow; 

      return fileIndex;  
    } 
} 

Answer 1

http://msdn.microsoft.com/en-us/library/ms804359.aspx

第三个参数是一个指向您分配一个缓冲区，以保存返回值。 在这种情况下，基于所述5个参数的值是一个指向FILE_INTERNAL_INFORMATION结构

http://msdn.microsoft.com/en-us/library/ms791535.aspx

则然后从下一行的进objectIDInfo deferenced。

Answer 2

你需要像这样（未测试）：

public ulong ApproachA() 
{ 
    WinAPI.IO_STATUS_BLOCK iostatus=new WinAPI.IO_STATUS_BLOCK(); 

    // allocate space for _FILE_INTERNAL_INFORMATION 
    IntPtr memPtr = 
     Marshal.AllocHGlobal(Marshal.SizeOf(
      typeof(WinAPI._FILE_INTERNAL_INFORMATION))); 

    // note the size 
    int structSize = Marshal.SizeOf(WinAPI._FILE_INTERNAL_INFORMATION); 

    FileInfo fi=new FileInfo(@"C:\Temp\testfile.txt"); 

    FileStream fs = fi.Open(FileMode.Open,FileAccess.Read,FileShare.ReadWrite); 

    // pass the address of the _FILE_INTERNAL_INFORMATION block 
    // and this function will fill it in 
    IntPtr res 
     = WinAPI.NtQueryInformationFile(fs.Handle, ref iostatus, 
     memPtr, (uint)structSize, 
     WinAPI.FILE_INFORMATION_CLASS.FileInternalInformation); 

    // dereference and cast it to the correct type 
    // so our managed code can get the data out of it 
    WinAPI._FILE_INTERNAL_INFORMATION objectIDInfo = 
     (WinAPI._FILE_INTERNAL_INFORMATION)Marshal.PtrToStructure(memPtr, 
     typeof(WinAPI._FILE_INTERNAL_INFORMATION)); 

    fs.Close(); 

    // make a local copy of just the thing we want 
    long IndexNumber = objectIDInfo.IndexNumber; 

    // free what we allocated 
    Marshal.FreeHGlobal(memPtr);  

    return IndexNumber; 
}