C# 调用IExtractMage.GetLocation（）失败，出现NullReference异常

我有以下代码来导入IExtractMage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，为了简洁起见，这里不提它。我的目的是使用shellfolder访问文件的缩略图。这是我检索缩略图的代码。但是我对IExtractMage.GetLocation（）的调用失败，出现NullReference异常，声明

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

其他信息：对象引用未设置为对象的实例。“

有人能帮我确定我遗漏了什么吗

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;
} 

---

我看到在步骤4中，没有正确检索到pidlMediaFile，在ParseDisplayName（）调用之后，它的值仍然是0。这就是问题的开始。我不确定为什么没有检索文件名的pidl，而成功地检索了文件父文件夹的pidl。

看起来

extractImage

正在从

GetUIObjectOf（）

返回

null

。尝试检查

GetUIObjectOf（）

中的

HRESULT

返回值，以找出返回

null

值的原因

编辑：

发件人：

---

根据，第三个参数是

[in]

参数，而不是

[out]

参数。

似乎

extractImage

是从

GetUIObjectOf（）

返回的

null

？我同意。基于API调用，您是否觉得extractImage为null的原因很明显？

GetUiObjectOf（）

返回一个

HRESULT

错误代码。你可以检查一下。我得到这个错误值：-2147024809（十六进制等于FFFFFFFF80070057）。但是我在任何地方都找不到这个错误代码的定义。现在，我不确定我是否使用正确的参数以正确的顺序进行shell API调用。下面是我正在做的：现在，我不确定是否使用正确的参数以正确的顺序进行shell API调用。以下是我正在做的：1。获取桌面文件夹。2.使用桌面文件夹获取媒体文件夹的idl。3.使用刚刚检索到的idl获取媒体文件夹的外壳对象。4.使用媒体文件夹的shell对象调用GetUIObjectOf（）并传递刚检索到的idl。我已删除此步骤列表中的步骤shellMediaFolder.ParseDisplayName（），因为在整个步骤序列中，这似乎是一个错误的步骤。但我还是得到了同样的错误。谷歌“hresult”，你会找到答案：

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;
}

E_INVALIDARG    One or more arguments are not valid 0x80070057