Enumerate Media Foundation Transforms

The MediaFoundationReader and MediaFoundationEncoder classes in NAudio make use of any available Media Foundation Transforms (MFTs) installed on your computer. It can be useful to enumerate the available audio-related MFTs.

There are three categories of audio MFT - effects, decoders and encoders. A decoder allows you to decode audio compressed in different formats to PCM. An encoder allows you to encode PCM audio into compressed formats. An effect modifies audio in some way (e.g. resampling).

You can use MediaFoundationApi.EnumerateTransforms to explore what's available:

MediaFoundationApi.Startup();

var effects = MediaFoundationApi.EnumerateTransforms(MediaFoundationTransformCategories.AudioEffect);
var decoders = MediaFoundationApi.EnumerateTransforms(MediaFoundationTransformCategories.AudioDecoder);
var encoders = MediaFoundationApi.EnumerateTransforms(MediaFoundationTransformCategories.AudioEncoder);

These return an IEnumerable<MfActivate>. The MfActivate wrapper provides methods to explore the attributes of each transform:

private string DescribeMft(MfActivate mft)
{
    int attributeCount = mft.AttributeCount;
    var sb = new StringBuilder();
    for (int n = 0; n < attributeCount; n++)
    {
        AddAttribute(mft, n, sb);
    }
    return sb.ToString();
}

private static void AddAttribute(MfActivate mft, int index, StringBuilder sb)
{
    var variantPtr = Marshal.AllocHGlobal(Marshal.SizeOf<PropVariant>());
    try
    {
        mft.GetAttributeByIndex(index, out var key, variantPtr);
        var value = Marshal.PtrToStructure<PropVariant>(variantPtr);
        var propertyName = FieldDescriptionHelper.Describe(typeof(MediaFoundationAttributes), key);
        if (key == MediaFoundationAttributes.MFT_INPUT_TYPES_Attributes ||
            key == MediaFoundationAttributes.MFT_OUTPUT_TYPES_Attributes)
        {
            var types = mft.GetBlobAsArrayOf<MftRegisterTypeInfo>(key);
            sb.AppendFormat("{0}: {1} items:", propertyName, types.Length);
            sb.AppendLine();
            foreach (var t in types)
            {
                sb.AppendFormat("    {0}-{1}",
                    FieldDescriptionHelper.Describe(typeof(MediaTypes), t.MajorType),
                    FieldDescriptionHelper.Describe(typeof(AudioSubtypes), t.SubType));
                sb.AppendLine();
            }
        }
        else if (key == MediaFoundationAttributes.MF_TRANSFORM_CATEGORY_Attribute)
        {
            sb.AppendFormat("{0}: {1}", propertyName,
                FieldDescriptionHelper.Describe(typeof(MediaFoundationTransformCategories), (Guid)value.Value));
            sb.AppendLine();
        }
        else if (value.DataType == (VarEnum.VT_VECTOR | VarEnum.VT_UI1))
        {
            var b = (byte[])value.Value;
            sb.AppendFormat("{0}: Blob of {1} bytes", propertyName, b.Length);
            sb.AppendLine();
        }
        else
        {
            sb.AppendFormat("{0}: {1}", propertyName, value.Value);
            sb.AppendLine();
        }
    }
    finally
    {
        PropVariant.Clear(variantPtr);
        Marshal.FreeHGlobal(variantPtr);
    }
}

Here's an example output for an MFT effect. In this case, the Resampler which is a very useful MFT for changing sample rates:

Audio Effect
Name: Resampler MFT
Input Types: 2 items:
    Audio-PCM
    Audio-IEEE floating-point
Class identifier: f447b69e-1884-4a7e-8055-346f74d6edb3
Output Types: 2 items:
    Audio-PCM
    Audio-IEEE floating-point
Transform Flags: 1
Transform Category: Audio Effect

Here's an example output for a decoder. Windows 10 and above includes an Opus audio decoder:

Audio Decoder
Name: Microsoft Opus Audio Decoder MFT
Input Types: 1 items:
    Audio-Opus
Class identifier: 63e17c10-2d43-4c42-8fe3-8d8b63e46a6a
Output Types: 1 items:
    Audio-IEEE floating-point
Transform Flags: 1
Transform Category: Audio Decoder

And an encoder. Windows 10 and above includes a FLAC encoder:

Audio Encoder
Name: Microsoft FLAC Audio Encoder MFT
Input Types: 1 items:
    Audio-PCM
Class identifier: 128509e9-c44e-45dc-95e9-c255b8f466a6
Output Types: 1 items:
    Audio-FLAC
Transform Flags: 1
Transform Category: Audio Encoder

You can also activate a transform directly from the MfActivate object. For example, to create an MfTransform wrapper:

using var transform = mft.ActivateTransform();