ProcessLoopbackInterop.cs

using System.Runtime.InteropServices;
using NAudio.CoreAudioApi.Interfaces;

namespace MultiAudioRouter.Core;

/// <summary>
/// Windows Process Loopback API の P/Invoke 定義。
/// 仮想デバイス "VAD\Process_Loopback" 経由で IAudioClient を取得し、
/// PROCESS_LOOPBACK_PARAMS で対象 PID とツリー INCLUDE/EXCLUDE モードを指定する。
/// 必要 OS: Windows 10 build 20348 (21H2) / Windows 11 以降。
/// </summary>
internal static class ProcessLoopbackInterop
{
    public const string VirtualDevicePath = "VAD\\Process_Loopback";

    public const uint AUDCLNT_STREAMFLAGS_LOOPBACK     = 0x00020000;
    public const uint AUDCLNT_STREAMFLAGS_EVENTCALLBACK = 0x00040000;

    public const int AUDCLNT_SHAREMODE_SHARED = 0;

    public const int AUDIOCLIENT_ACTIVATION_TYPE_PROCESS_LOOPBACK = 1;
    public const int PROCESS_LOOPBACK_MODE_INCLUDE_TARGET_PROCESS_TREE = 0;
    public const int PROCESS_LOOPBACK_MODE_EXCLUDE_TARGET_PROCESS_TREE = 1;

    // PROPVARIANT VT
    public const ushort VT_BLOB = 0x41;

    // IID_IAudioClient
    public static readonly Guid IID_IAudioClient = new("1CB9AD4C-DBFA-4c32-B178-C2F568A703B2");
    public static readonly Guid IID_IAudioCaptureClient = new("C8ADBD64-E71E-48a0-A4DE-185C395CD317");

    [Flags]
    public enum AudioClientBufferFlags : uint
    {
        None = 0,
        DataDiscontinuity = 0x1,
        Silent = 0x2,
        TimestampError = 0x4,
    }

    /// <summary>自前 IAudioCaptureClient (NAudio のは internal なので)。</summary>
    [Guid("C8ADBD64-E71E-48a0-A4DE-185C395CD317")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IAudioCaptureClient
    {
        [PreserveSig]
        int GetBuffer(out IntPtr ppData, out int pNumFramesToRead, out AudioClientBufferFlags pdwFlags,
                      out long pu64DevicePosition, out long pu64QPCPosition);
        [PreserveSig]
        int ReleaseBuffer(int numFramesRead);
        [PreserveSig]
        int GetNextPacketSize(out int pNumFramesInNextPacket);
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct AUDIOCLIENT_PROCESS_LOOPBACK_PARAMS
    {
        public uint TargetProcessId;
        public int ProcessLoopbackMode;
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct AUDIOCLIENT_ACTIVATION_PARAMS
    {
        public int ActivationType;
        public AUDIOCLIENT_PROCESS_LOOPBACK_PARAMS ProcessLoopbackParams;
    }

    /// <summary>
    /// PROPVARIANT (x64 想定で 24 バイト) を BLOB として手動構築するための領域。
    /// </summary>
    [StructLayout(LayoutKind.Sequential, Size = 24)]
    public struct PROPVARIANT_BLOB
    {
        public ushort vt;
        public ushort wReserved1;
        public ushort wReserved2;
        public ushort wReserved3;
        public uint cbSize;     // BLOB.cbSize
        // 4 bytes pad on x64
        public uint pad;
        public IntPtr pBlobData; // BLOB.pBlobData
    }

    [Guid("72A22D78-CDE4-431D-B8CC-843A71199B6D")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IActivateAudioInterfaceAsyncOperation
    {
        [PreserveSig]
        int GetActivateResult(out int activateResult,
            [MarshalAs(UnmanagedType.IUnknown)] out object activatedInterface);
    }

    [Guid("41D949AB-9862-444A-80F6-C261334DA5EB")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IActivateAudioInterfaceCompletionHandler
    {
        [PreserveSig]
        int ActivateCompleted(IActivateAudioInterfaceAsyncOperation activateOperation);
    }

    [DllImport("Mmdevapi.dll", ExactSpelling = true, PreserveSig = false)]
    public static extern void ActivateAudioInterfaceAsync(
        [MarshalAs(UnmanagedType.LPWStr)] string deviceInterfacePath,
        ref Guid riid,
        IntPtr activationParams,
        IActivateAudioInterfaceCompletionHandler completionHandler,
        out IActivateAudioInterfaceAsyncOperation activationOperation);

    /// <summary>
    /// ManualResetEvent ベースの完了ハンドラ。同期待ちで IAudioClient を受け取る。
    /// </summary>
    [ComVisible(true)]
    public sealed class CompletionHandler : IActivateAudioInterfaceCompletionHandler
    {
        public ManualResetEventSlim Done { get; } = new(false);

        public int ActivateCompleted(IActivateAudioInterfaceAsyncOperation op)
        {
            Done.Set();
            return 0; // S_OK
        }
    }

    /// <summary>
    /// IAudioClient (NAudio の interface) を取得するヘルパー。
    /// 失敗時は HRESULT を含む Exception を投げる。
    /// </summary>
    public static IAudioClient ActivateProcessLoopbackAudioClient(uint pid, bool excludeMode)
    {
        var clientParams = new AUDIOCLIENT_ACTIVATION_PARAMS
        {
            ActivationType = AUDIOCLIENT_ACTIVATION_TYPE_PROCESS_LOOPBACK,
            ProcessLoopbackParams = new AUDIOCLIENT_PROCESS_LOOPBACK_PARAMS
            {
                TargetProcessId = pid,
                ProcessLoopbackMode = excludeMode
                    ? PROCESS_LOOPBACK_MODE_EXCLUDE_TARGET_PROCESS_TREE
                    : PROCESS_LOOPBACK_MODE_INCLUDE_TARGET_PROCESS_TREE,
            },
        };

        var paramsSize = Marshal.SizeOf<AUDIOCLIENT_ACTIVATION_PARAMS>();
        var paramsPtr = Marshal.AllocCoTaskMem(paramsSize);
        var propvariantPtr = Marshal.AllocCoTaskMem(Marshal.SizeOf<PROPVARIANT_BLOB>());

        try
        {
            Marshal.StructureToPtr(clientParams, paramsPtr, false);

            var pv = new PROPVARIANT_BLOB
            {
                vt = VT_BLOB,
                cbSize = (uint)paramsSize,
                pBlobData = paramsPtr,
            };
            Marshal.StructureToPtr(pv, propvariantPtr, false);

            var iid = IID_IAudioClient;
            var handler = new CompletionHandler();
            ActivateAudioInterfaceAsync(VirtualDevicePath, ref iid, propvariantPtr, handler, out var op);

            if (!handler.Done.Wait(TimeSpan.FromSeconds(5)))
                throw new TimeoutException("ActivateAudioInterfaceAsync の完了を 5 秒以内に受け取れませんでした。");

            var hr = op.GetActivateResult(out var activateResult, out var iface);
            if (hr < 0)
                throw Marshal.GetExceptionForHR(hr) ?? new InvalidOperationException($"GetActivateResult HRESULT 0x{hr:X8}");
            if (activateResult < 0)
                throw Marshal.GetExceptionForHR(activateResult)
                    ?? new InvalidOperationException($"アクティベート失敗 HRESULT 0x{activateResult:X8}");

            return (IAudioClient)iface;
        }
        finally
        {
            Marshal.FreeCoTaskMem(propvariantPtr);
            Marshal.FreeCoTaskMem(paramsPtr);
        }
    }
}