Unable to use MFEnumDeviceSources().

[Steph55]

I tried using the function MFEnumDeviceSources() - reference issue #22 - with many different methods, but it seems quite difficult in C#. How should we proceed to access the contents of the out parameter pppSourceActivate?

I even tried making my own function signature, without success:

[DllImport("mf", ExactSpelling = true)] public static extern HRESULT MFEnumDeviceSources( ref IMFAttributes pAttributes, [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 2)] out IMFActivate[] pppSourceActivate, out uint pcSourceActivate);

Maybe with unsafe code?

Thanks a lot!

[smourier]

Hi,

You never need unsafe code. And DirectN was designed with that idea in mind.

There was a bug in the MFEnumDeviceSources definition. The first parameter isn't ref. I have updated DirectN.

As for calling the method, I've also added an extension in IMFAttributesExtensions.cs that explains how to do it:

public static IEnumerable<IComObject<IMFActivate>> EnumDeviceSources(this IComObject<IMFAttributes> input) => EnumDeviceSources(input?.Object).Select(a => new ComObject<IMFActivate>(a));
public static IEnumerable<IMFActivate> EnumDeviceSources(this IMFAttributes input)
{
    if (input == null)
        throw new ArgumentNullException(nameof(input));

    Functions.MFEnumDeviceSources(input, out var array, out var count).ThrowOnError();
    for (var i = 0; i < count; i++)
    {
        var ptr = Marshal.ReadIntPtr(array, i * IntPtr.Size);
        var activate = (IMFActivate)Marshal.GetObjectForIUnknown(ptr);
        yield return activate;
        Marshal.Release(ptr);
    }

    Marshal.FreeCoTaskMem(array);
}

So you can call it like this for example (equivalent of code here: https://docs.microsoft.com/en-us/windows/win32/api/mfidl/nf-mfidl-mfenumdevicesources#examples):

MFFunctions.MFStartup();
var atts = MFFunctions.MFCreateAttributes();
atts.Set(MFConstants.MF_DEVSOURCE_ATTRIBUTE_SOURCE_TYPE, MFConstants.MF_DEVSOURCE_ATTRIBUTE_SOURCE_TYPE_VIDCAP_GUID);

foreach (var src in atts.EnumDeviceSources())
{
    var source = src.ActivateObject<IMFMediaSource>();
    source.Object.GetCharacteristics(out var c);
}
...