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Custom .NET –> COM Interop

Jan 22

Written by: Jeremiah Morrill
1/22/2009 8:44 PM 

I'm having many issues with how .NET handles COM objects while writing this EVR custom presenter.  I'm getting around it by writing my own COM wrapper for the 'trouble making' COM objects (most notibly IMFSample).  Here is the base class for the custom COM wrapper I am going to use.  It's a bit more boiler plate, but it will be well worth it :).

 

abstract public class Unknown
{
    private readonly IntPtr m_comPointer;

    private delegate int AddRefDelegate(IntPtr comPointer);
    private delegate int ReleaseDelegate(IntPtr comPointer);
    private delegate int QueryInterfaceDelegate(IntPtr comPointer, ref Guid riid, IntPtr ppvObject);

    private AddRefDelegate AddRefMethod;
    private ReleaseDelegate ReleaseMethod;
    private QueryInterfaceDelegate QueryInterfaceMethod;

    protected Unknown(IntPtr comPointer)
    {
        m_comPointer = comPointer;
        SetupIUnknownMethods();
    }

    private void SetupIUnknownMethods()
    {
        /* Read the COM v-table pointer */
        IntPtr pVtable = Marshal.ReadIntPtr(m_comPointer);

        const int ADDREF_VTABLE_INDEX = 1;
        const int RELEASE_VTABLE_INDEX = 2;
        const int QUERYINTERFACE_VTABLE_INDEX = 0;

        /* Get the function pointer */
        IntPtr pFunc = Marshal.ReadIntPtr(pVtable, ADDREF_VTABLE_INDEX * IntPtr.Size);

        /* Cast the function pointer to a .NET delegate */
        AddRefMethod = (AddRefDelegate)Marshal.GetDelegateForFunctionPointer(pFunc, typeof(AddRefDelegate));

        /* Rinse and repeat */
        pFunc = Marshal.ReadIntPtr(pVtable, RELEASE_VTABLE_INDEX * IntPtr.Size);

        ReleaseMethod = (ReleaseDelegate)Marshal.GetDelegateForFunctionPointer(pFunc, typeof(ReleaseDelegate));

        pFunc = Marshal.ReadIntPtr(pVtable, QUERYINTERFACE_VTABLE_INDEX * IntPtr.Size);

        QueryInterfaceMethod = (QueryInterfaceDelegate)Marshal.GetDelegateForFunctionPointer(pFunc, typeof(QueryInterfaceDelegate));
    }

    public int AddRef()
    {
        return AddRefMethod(m_comPointer);
    }

    public int Release()
    {
        return ReleaseMethod(m_comPointer);
    }

    public int QueryInterface(IntPtr comPointer, ref Guid riid, IntPtr ppvObject)
    {
        return QueryInterfaceMethod(comPointer, ref riid, ppvObject);
    }

    public IntPtr ComPointer
    {
        get { return m_comPointer; }
    }
}

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12 comment(s) so far...

Re: Custom .NET –> COM Interop

Hey Jer, keep up the great work. There is some good code you may be interested in at: babgvant.com/files/folders/evrplay/category1354.aspx

By Jarrett Vance on   1/25/2009 7:51 PM

Re: Custom .NET –> COM Interop

Now you've got the idea. V-Table and function pointers is the way to go. A good buddy of mine does his C# filters using this same approach its much cleaner and more efficient than allowing the runtime to Marshal COM objects. You also avoid STA/MTA cross-thread "no interface" limitations.

It works nice to do it in generics too, and then make pseudo directshow interfaces.. eg;

class ComBase
{
internal IntPtr _ptr;

// Helper method to create a delegate off the pointer
internal static T CreateDelegate(IntPtr ptr, Type interfaceType, string MethodName)
{
int relativeMethodIndex = -1;
System.Reflection.MethodInfo[] ar = interfaceType.GetMethods();
for (int i = 0; i (ptr, IMemInputPinType, "GetAllocator");
_NotifyAllocator = CreateDelegate(ptr, IMemInputPinType, "NotifyAllocator");
_GetAllocatorRequirements = CreateDelegate(ptr, IMemInputPinType, "GetAllocatorRequirements");
_Receive = CreateDelegate(ptr, IMemInputPinType, "Receive");
_ReceiveMultiple = CreateDelegate(ptr, IMemInputPinType, "ReceiveMultiple");
_ReceiveCanBlock = CreateDelegate(ptr, IMemInputPinType, "ReceiveCanBlock");
}

#region IMemInputPin Members
delegate HRESULT GetAllocatorDelegate(IntPtr ptr, out IMemAllocator ppAllocator);
public HRESULT GetAllocator(out IMemAllocator ppAllocator)
{
return _GetAllocator(_ptr, out ppAllocator);
}

delegate HRESULT NotifyAllocatorDelegate(IntPtr ptr, IntPtr pAllocator, bool bReadOnly);
public HRESULT NotifyAllocator(IntPtr pAllocator, bool bReadOnly)
{
return _NotifyAllocator(_ptr, pAllocator, bReadOnly);
}

delegate HRESULT GetAllocatorRequirementsDelegate(IntPtr ptr, out AllocatorProperties pProps);
public HRESULT GetAllocatorRequirements(out AllocatorProperties pProps)
{
return _GetAllocatorRequirements(_ptr, out pProps);
}

delegate HRESULT ReceiveDelegate(IntPtr ptr, IMediaSample pSample);
public HRESULT Receive(IMediaSample pSample)
{
return _Receive(_ptr, pSample);
}

delegate HRESULT ReceiveMultipleDelegate(IntPtr ptr, IMediaSample[] pSamples, int nSamples, out int nSamplesProcessed);
public HRESULT ReceiveMultiple(IMediaSample[] pSamples, int nSamples, out int nSamplesProcessed)
{
return _ReceiveMultiple(_ptr, pSamples, nSamples, out nSamplesProcessed);
}

delegate HRESULT ReceiveCanBlockDelegate(IntPtr ptr);
public HRESULT ReceiveCanBlock()
{
return _ReceiveCanBlock(_ptr);
}

#endregion
}

The C# TS source filter/demux mentioned the other day use the lame .NET automatic Marshalling -- but I hadn't come up against any problems with it, (if it ain't broke dont fix it etc.)

Good luck!

By Sichbo on   2/1/2009 8:04 AM

Re: Custom .NET –> COM Interop

heh looks like theres a bug in this comment thing... seems to have cropped the middle portion of that code :) eh well

By Sichbo on   2/1/2009 8:10 AM

Re: Custom .NET –> COM Interop

Thats good stuff Sichbo!

I won't claim that the DNN has the best blog system...

If you'd like to email me (I'd love to hear more!), my email is jeremiah.morrill [at] gmail dot com

-Jer

By Jeremiah Morrill on   2/1/2009 10:43 AM

Re: Custom .NET –> COM Interop

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