There is something wrong with the type definition of IThumbnailProvider for COM interop purposes

[catmanjan]

I was trying to use IThumbnailProvider like this other person was: dahall/WinClassicSamplesCS#6

I found a similar problem where my cloud provider sync root was triggering the Initialize function (from my IInitializeWithItem implemention) but not GetThumbnail.

I then tried writing my own interface like so:

    [ComVisible(true), Guid("e357fccd-a995-4576-b01f-234630154e96"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IThumbnailProvider
    {
        HRESULT GetThumbnail(int cx, out IntPtr hBitmap, out WTS_ALPHATYPE bitmapType);
    }

And it just worked, so I think maybe there is something wrong with the Vanara version.

Could it be the type of hBitmap is not casting correctly?

[catmanjan]

Ok I just brute forced the solution... This works:

    [PInvokeData("thumbcache.h", MSDNShortId = "55c4739a-4835-4f53-a435-804ddf06ffcf")]
    [ComImport, Guid("e357fccd-a995-4576-b01f-234630154e96"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IThumbnailProvider
    {
        [PreserveSig]
        HRESULT GetThumbnail(uint cx, out IntPtr phbmp, out WTS_ALPHATYPE pdwAlpha);
    }

I removed CoClass and changed the type of phbmp, if I readd either it stops working again

[dahall]

I can fix that definition, but I'd love to test it the way you are. Right now, I can use it by getting an IShellItem instance, then using BindToHandler to get the IThumbnailProvider and then calling GetThumbnail. That seems to work using the current definition. You must be doing something different. Will you send me a code snippet of how you exercise GetThumbnail?

[dahall]

I just ran this code with success too.

IShellItem? shi = GetShellItemForPath(TestCaseSources.ImageFile);
var istr = shi!.BindToHandler<System.Runtime.InteropServices.ComTypes.IStream>(null, BHID.BHID_Stream);

IThumbnailProvider tp = (IThumbnailProvider)new PhotoThumbnailProvider();
(tp as IInitializeWithStream)?.Initialize(istr, STGM.STGM_READ);
if (tp.GetThumbnail(256, out var hBmp, out _).Succeeded && !hBmp.IsInvalid)
   Console.Write("Success");

[catmanjan]

I think the best way to check it yourself is to try https://github.com/dahall/WinClassicSamplesCS/blob/nullableenabled/CloudMirror/ThumbnailProvider.cs

If you run the cloudmirror project you should see thumbnails get generated for placeholder files but this doesn't currently happen because of this issue - if you swap the type of IThumbnailProvider it should start serving thumbnails