InvalidCastException when calling COM interface on Windows Mobile.

Question

InvalidCastException when calling COM interface on Windows Mobile.

I am trying to build a DirectShow source filter on Windows Mobile 5 based on the Generic Sample Source Filter (GSSF) sample from DirectShowLib. It uses C# configuration and callback functions with a C++ DirectShow filter. I have this working in .Net CF on Windows XP but when I run it on my Windows Mobile device I get an InvalidCastException when getting the configuration interface on the filter's output pin.

Please can someone suggest a solution?

C# Code

public void Test()
{
    int hr = 0;
    IBaseFilter ipsb = (IBaseFilter)new GenericSampleSourceFilter();
    IPin ipin = null;
    Log("FindPin");
    hr = ipsb.FindPin("1", out ipin);
    DsError.ThrowExceptionForHR( hr );
    string id = null;
    hr = ipin.QueryId(out id);
    DsError.ThrowExceptionForHR( hr );
    Log("Id=" + id);
    IGenericSampleConfig icfg = (IGenericSampleConfig)ipin; // InvalidCastException on WM5
}

[ComImport, Guid("6F7BCF72-D0C2-4449-BE0E-B12F580D056D")]
public class GenericSampleSourceFilter
{
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
Guid("33B9EE57-1067-45fa-B12D-C37517F09FC0")]
public interface IGenericSampleCB
{
    [PreserveSig]
    int SampleCallback(IMediaSample pSample);
}

[Guid("CE50FFF9-1BA8-4788-8131-BDE7D4FFC27F"),
InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IGenericSampleConfig
{
    [PreserveSig]
    int SetMediaTypeFromBitmap(BitmapInfoHeader bmi, long lFPS);

    [PreserveSig]
    int SetMediaType([MarshalAs(UnmanagedType.LPStruct)] AMMediaType amt);

    [PreserveSig]
    int SetMediaTypeEx([MarshalAs(UnmanagedType.LPStruct)] AMMediaType amt, int lBufferSize);

    [PreserveSig]
    int SetBitmapCB(IGenericSampleCB pfn);
}

C++ code

// {6F7BCF72-D0C2-4449-BE0E-B12F580D056D}
DEFINE_GUID(CLSID_GenericSampleSourceFilter, 
0x6f7bcf72, 0xd0c2, 0x4449, 0xbe, 0xe, 0xb1, 0x2f, 0x58, 0xd, 0x5, 0x6d);

// {CE50FFF9-1BA8-4788-8131-BDE7D4FFC27F}
DEFINE_GUID(IID_IGenericSampleConfig, 
0xce50fff9, 0x1ba8, 0x4788, 0x81, 0x31, 0xbd, 0xe7, 0xd4, 0xff, 0xc2, 0x7f);

// {33B9EE57-1067-45fa-B12D-C37517F09FC0}
DEFINE_GUID(IID_IGenericSampleCB, 
0x33b9ee57, 0x1067, 0x45fa, 0xb1, 0x2d, 0xc3, 0x75, 0x17, 0xf0, 0x9f, 0xc0);

DECLARE_INTERFACE_(IGenericSampleCB, IUnknown) {
    STDMETHOD(SampleCallback)(THIS_
        IMediaSample *pSample
        ) PURE;
};

DECLARE_INTERFACE_(IGenericSampleConfig, IUnknown) {

    STDMETHOD(SetMediaTypeFromBitmap) (THIS_
                BITMAPINFOHEADER *bmi,
                LONGLONG lFPS
             ) PURE;

    STDMETHOD(SetMediaType) (THIS_
                AM_MEDIA_TYPE *amt
             ) PURE;

    STDMETHOD(SetMediaTypeEx) (THIS_
                AM_MEDIA_TYPE *amt,
                long lBufferSize
             ) PURE;

    STDMETHOD(SetBitmapCB) (THIS_
                IGenericSampleCB *pfn
             ) PURE;
};