IExtractImage：提取方法“对象引用未设置为对象的实例”。错误

Question

我想开发办公文档的图像库，它将创建办公文档的缩略图。

我收到了http://msdn.microsoft.com/en-us/library/aa289172.aspx

的代码

我得到的代码是在vb.net中我转换为c＃。

 private bool _getThumbNail(string file, IntPtr pidl, IShellFolder item)
        {
            IntPtr hBmp = IntPtr.Zero;
            IExtractImage extractImage = null;
            try
            {
                string pidlPath = PathFromPidl(pidl);
                if (Path.GetFileName(pidlPath).ToUpper().Equals(Path.GetFileName(file).ToUpper()))
                {
                    IUnknown iunk = null;
                    int prgf = 0;
                    Guid iidExtractImage = new Guid("BB2E617C-0920-11d1-9A0B-00C04FC2D6C1");
                    item.GetUIObjectOf(IntPtr.Zero, 1, ref pidl, ref iidExtractImage, ref prgf, ref iunk);
                    extractImage = (IExtractImage)iunk;
                    if (extractImage != null)
                    {
                        Console.WriteLine("Got an IExtractImage object!");
                        SIZE sz = new SIZE();
                        sz.cx = DesiredSize.Width + 1;
                        sz.cy = DesiredSize.Height + 1;
                        StringBuilder location = new StringBuilder(260, 260);
                        int priority = 0;
                        int requestedColourDepth = 32;
                        EIEIFLAG flags = EIEIFLAG.IEIFLAG_ASPECT | EIEIFLAG.IEIFLAG_SCREEN | EIEIFLAG.IEIFLAG_ASYNC;
                        int uFlags = (int)flags;
                        try
                        {
                            extractImage.GetLocation(location, location.Capacity, ref priority, ref sz, requestedColourDepth, ref uFlags);
                            //if(hBmp != IntPtr.Zero)
                            extractImage.Extract(ref hBmp);
                        }
                        catch (System.Runtime.InteropServices.COMException ex)
                        {
                            Console.Write(ex.Message);
                        }
                        if (hBmp != IntPtr.Zero)
                        {
                            _thumbNail = Bitmap.FromHbitmap(hBmp);
                        }
                        Marshal.ReleaseComObject(extractImage);
                        extractImage = null;
                    }
                    return true;
                }
                else
                {
                    return false;
                }
            }
            catch (Exception ex)
            {
                if (hBmp != IntPtr.Zero)
                {
                    UnmanagedMethods.DeleteObject(hBmp);
                }
                if (extractImage != null)
                {
                    Marshal.ReleaseComObject(extractImage);
                }
                throw ex;
            }
        }

这是我在 extractImage.Extract（ref hBmp）;

面临问题的功能

任何人都可以帮助我，我正在使用VS2010和OS Vista 32位。