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我有以下代码来导入 IExtractImage 接口。

[ComImport]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("BB2E617C-0920-11d1-9A0B-00C04FC2D6C1")]
public interface IExtractImage
{
  [PreserveSig]
   Int32 GetLocation([MarshalAs(UnmanagedType.LPWStr)] out StringBuilder pszPathBuffer, 
     int cch, 
     ref int pdwPriority, 
     SIZE prgSize, 
     int dwRecClrDepth, 
     ref int pdwFlags);

  [PreserveSig]
    Int32 Extract(out IntPtr phBmpThumbnail);
}

我还导入了 IShellFolder，为简洁起见，我在这里不提。我的目的是使用 shellfolder 访问文件的缩略图。所以这是我检索缩略图的代码。但是我对 IExtractImage.GetLocation() 的调用因 NullReference 异常而失败，说明

“在 CCDash.exe 中发生了‘System.NullReferenceException’类型的异常，但未在用户代码中处理

附加信息：对象引用未设置为对象的实例。”

有人可以帮我确定我错过了什么吗？

public Bitmap GetThumbNail(string mediaFileName)
{
  IShellFolder shellDesktop;
  SHGetDesktopFolder(out shellDesktop);

  IntPtr pidlRoot;
  uint attribute = 0;
  string mediaPath = "C:\\Users\\<user>\\Videos"; // I have hard-coded the path for now
  uint pchEaten = 0;
  // Get the pidl of the media folder
  shellDesktop.ParseDisplayName(IntPtr.Zero, IntPtr.Zero, mediaPath, ref pchEaten, out pidlRoot, ref attribute);

  Guid mediaFolderGuid = new Guid("000214E6-0000-0000-C000-000000000046");
  shellDesktop.BindToObject(pidlRoot, IntPtr.Zero, mediaFolderGuid, out shellMediaFolder);


  IntPtr pidlMediaFolder;
  // Get the pidl of the media file
  shellMediaFolder.ParseDisplayName(IntPtr.Zero, IntPtr.Zero, mediaFileName, ref pchEaten, out pidlMediaFolder, ref attribute);

  Guid mediaFileImgGuid = new Guid("BB2E617C-0920-11d1-9A0B-00C04FC2D6C1");      
  uint rfgRes = 0;
  IExtractImage extractImage;
  shellMediaFolder.GetUIObjectOf(IntPtr.Zero, 1, out pidlMediaFolder, mediaFileImgGuid, ref rfgRes, out extractImage);

  SIZE size = new SIZE
  {
    cx = 40,
    cy = 40
  };

  int flags = 0x40 | 0x40;
  StringBuilder location = new StringBuilder(260, 260);
  int priority = 0;
  int requestedColourDepth = 0x20;
  IntPtr hBmp = IntPtr.Zero;
  // Now get the image
  extractImage.GetLocation(out location, location.Capacity, ref priority, size, requestedColourDepth, ref flags);
  extractImage.Extract(out hBmp);

  Bitmap thumbnail = Image.FromHbitmap(hBmp);

  return thumbnail;
}

编辑：

我现在修改了我的代码，如下所示。与第一个版本没有太大区别，但有更多的错误处理和更好的文档和变量名，可以帮助我们更好地理解我的代码。这是修改后的代码：

public Bitmap GetThumbNail(string mediaFileName)
{
  Bitmap thumbnail = null;

  //Step 1: Use SHGetDesktopFolder to get the desktop folder.
  IShellFolder shellDesktop;
  SHGetDesktopFolder(out shellDesktop);

  if (shellDesktop != null)
  {
    IntPtr pidlMediaFolder;
    try
    {
      uint attribute = 0;
      string mediaPath = Path.GetDirectoryName(mediaFileName);
      uint pchEaten = 0;
      // Step 2: Using the desktop's IShellFolder, pass the file's parent folder path name into ParseDisplayName to get its PIDL.
      shellDesktop.ParseDisplayName(IntPtr.Zero, IntPtr.Zero, mediaPath, ref pchEaten, out pidlMediaFolder, ref attribute);
    }
    catch (Exception)
    {
      Marshal.ReleaseComObject(shellDesktop);
      return null;
    }

    if (pidlMediaFolder != IntPtr.Zero)
    {
      Guid mediaFolderGuid = new Guid("000214E6-0000-0000-C000-000000000046");
      IShellFolder shellMediaFolder;
      // Step 3: Using the desktop's IShellFolder, pass the PIDL into the BindToObject method 
      // and get the IShellFolder interface of the file's parent folder.          
      try
      {
        shellDesktop.BindToObject(pidlMediaFolder, IntPtr.Zero, mediaFolderGuid, out shellMediaFolder);
      }
      catch (Exception)
      {
        Marshal.ReleaseComObject(shellDesktop);
        return null;
      }

      if (shellMediaFolder != null)
      {
        IntPtr pidlMediaFile;
        uint attribute = 0;
        uint pchEaten = 0;
        // Step 4: Using the parent folder's IShellFolder, pass the file name into ParseDisplayName to get its PIDL.            
        int ret = shellMediaFolder.ParseDisplayName(IntPtr.Zero, IntPtr.Zero, mediaFileName, ref pchEaten, out pidlMediaFile, ref attribute);

        Guid mediaFileImgGuid = new Guid("BB2E617C-0920-11d1-9A0B-00C04FC2D6C1");
        uint rfgRes = 0;
        IExtractImage extractImage;
        // Step 5: Using the parent folder's IShellFolder, pass the file's PIDL 
        // into the GetUIObjectOf. method to get the IExtractImage interface.            
        ret = shellMediaFolder.GetUIObjectOf(IntPtr.Zero, 1, out pidlMediaFile, mediaFileImgGuid, ref rfgRes, out extractImage);

        SIZE size = new SIZE
        {
          cx = 40,
          cy = 40
        };

        uint flags = 0x0200;
        StringBuilder location = new StringBuilder(260, 260);
        int priority = 0;
        int requestedColourDepth = 0x20;
        IntPtr hBmp = IntPtr.Zero;
        // Now get the image
        extractImage.GetLocation(out location, location.Capacity, ref priority, size, requestedColourDepth, ref flags);
        extractImage.Extract(out hBmp);

        thumbnail = Image.FromHbitmap(hBmp);
      }
    }
  }
  return thumbnail;
}

我看到在第 4 步，pidlMediaFile 没有被正确检索，并且在 ParseDisplayName() 调用之后它的值仍然是 0。这就是问题的开始。我不确定为什么文件名的 pidl 没有被检索到，而文件的父文件夹却成功检索到了它。

4

1 回答 1

0

看起来像是extractImage从null. GetUIObjectOf()尝试检查HRESULT返回值GetUIObjectOf()以找出返回值的原因null。

编辑：

来自http://msdn.microsoft.com/en-us/library/windows/desktop/aa378137%28v=vs.85%29.aspx：

E_INVALIDARG    One or more arguments are not valid 0x80070057

根据文档，第三个参数是一个[in]参数，而不是一个[out]参数。

于 2012-10-19T00:55:18.047 回答