Calling COM object methods from native pointers in C#

Question

In C#, is there an easy way to call a COM object method from a native pointer?

    [DllImport("d3d11.dll", CallingConvention = CallingConvention.StdCall)]
    private unsafe static extern int D3D11CreateDevice(
        void* arg0, int arg1, void* arg2, int   arg3, void* arg4,
        int   arg5, int arg6, void* arg7, void* arg8, void* arg9);

    public static void CreateDevice()
    {
        unsafe
        {
            IntPtr deviceOut;
            IntPtr immediateContextOut;
            int featureLevelRef;

            D3D11CreateDevice(
                (void*)IntPtr.Zero,
                1,
                (void*)IntPtr.Zero,
                32,
                (void*)IntPtr.Zero,
                0,
                7,
                &deviceOut,
                &featureLevelRef,
                &immediateContextOut);
        }
    }

In the code above, I get deviceOut whose type is ID3D11Device*. The ID3D11Device interface has a bunch of methods such as CreateBuffer().

Can I call some of these methods using deviceOut pointer? Thank you in advance.

You'd need to declare the interface. This has been done, avoid inventing that wheel. Both SlimDX and SharpDX are favored .NET wrappers for DX11. — Hans Passant
– Hans Passant, Commented Mar 8, 2013 at 23:08

JaredPar · Accepted Answer · 2013-03-08 22:10:37Z

2

If you have a pointer to a COM object you can convert it into an instance of the object via GetObjectForIUnknown.

IntPtr ptr = ...;
object unk = Marshal.GetObjectForIUnknown(ptr);
ID3D11Device dev = (ID3D11Device)unk;

From there you will be able to call the CreateBuffer and other methods

answered Mar 8, 2013 at 22:10

JaredPar

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1 Comment

Aaron Carter Over a year ago

I had to change my comment I made a little while ago. Oddly enough, when you create a DirectX COM object like an IDXGIFactory interface it will already have a ref count greater than zero! I suppose Windows already has one alive or something and it just passed you the pointer to it rather than creating a brand new one from scratch. This seems to be true for other DirectX COM objects as well, like I noticed IDXGIAdapterX interfaces may have a ref count of 4 to 6 already when you obtain them.

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