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POCall to IExtractImage.GetLocation() is failing with NullReference Exception
 

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  1. POCall to IExtractImage.GetLocation() is failing with NullReference Exception
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    <p>I have the following code to import the IExtractImage interface.</p> <pre><code>[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); } </code></pre> <p>I also have the IShellFolder imported, which I am not mentioning here for brevity. My intention is to access the thumbnail of a file using the shellfolder. So here's my code to retrieve the thumbnail. But my call to IExtractImage.GetLocation() is failing with a NullReference exception, stating</p> <p>"An exception of type 'System.NullReferenceException' occurred in CCDash.exe but was not handled in user code</p> <p>Additional information: Object reference not set to an instance of an object."</p> <p>Can someone please help me identify what is it that I am missing?</p> <pre><code>public Bitmap GetThumbNail(string mediaFileName) { IShellFolder shellDesktop; SHGetDesktopFolder(out shellDesktop); IntPtr pidlRoot; uint attribute = 0; string mediaPath = "C:\\Users\\&lt;user&gt;\\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; } </code></pre> <p>EDIT:</p> <p>I have now modified my code as below. Not very different from the first version, but with a little more error handling and better documentation and variable names that can help us understand my code better. Here's the modified code:</p> <pre><code>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; } </code></pre> <p>I see that at step 4, the pidlMediaFile is not retrieved correctly and it's value is still 0 after the ParseDisplayName() call. This is where the problem begins. I am not sure why the pidl for the filename is not retrieved whereas it is retrieved successfully for the file's parent folder.</p>
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    1. USMriganka
    1. USMriganka
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      1. POCall to IExtractImage.GetLocation() is failing with NullReference Exception
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      1. VTUpMod
    1. COIt would seem that `extractImage` is being returned `null` from `GetUIObjectOf()`?
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      1. POCall to IExtractImage.GetLocation() is failing with NullReference Exception
      1. USPeter Gluck
    2. COI agree. Does anything look obvious to you based on the API calls why the extractImage is null?
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      1. POCall to IExtractImage.GetLocation() is failing with NullReference Exception
      1. USMriganka
    3. CO`GetUiObjectOf()` returns an `HRESULT` error code. You might check that. http://msdn.microsoft.com/en-us/library/windows/desktop/bb775073%28v=vs.85%29.aspx
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      1. POCall to IExtractImage.GetLocation() is failing with NullReference Exception
      1. USPeter Gluck
 

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