DirectN

Direct Wrappers for .NET : DXGI, WIC, DirectX 9 to 12, Direct2D, Direct Write, Media Foundation, WASAPI, CodecAPI, GDI, Spatial Audio, DVD, Windows Media Player, etc.

You don't have to use the whole library (more than 6000 .cs files...), you can pick the files you want.

The code that you will find here allows you to port C/C++ code to C#, or to write C# code from scratch, more easily than with other existing libraries. For example, the exact same C# version of the "Capture screen using DirectX" code here: https://stackoverflow.com/questions/30021274/capture-screen-using-directx is this:

static HRESULT Direct3D9TakeScreenshots(uint adapter, int count)
{
    using (var d3d = new ComObject<IDirect3D9>(Functions.Direct3DCreate9(Constants.D3D_SDK_VERSION)))
    {
        var mode = new _D3DDISPLAYMODE();
        d3d.Object.GetAdapterDisplayMode(adapter, ref mode).ThrowOnError();

        var parameters = new _D3DPRESENT_PARAMETERS_();
        parameters.Windowed = true;
        parameters.BackBufferCount = 1;
        parameters.BackBufferHeight = mode.Height;
        parameters.BackBufferWidth = mode.Width;
        parameters.SwapEffect = _D3DSWAPEFFECT.D3DSWAPEFFECT_DISCARD;

        d3d.Object.CreateDevice(adapter, _D3DDEVTYPE.D3DDEVTYPE_HAL, IntPtr.Zero, Constants.D3DCREATE_SOFTWARE_VERTEXPROCESSING, ref parameters, out var dev).ThrowOnError();
        using (var device = new ComObject<IDirect3DDevice9>(dev))
        {
            dev.CreateOffscreenPlainSurface(mode.Width, mode.Height, _D3DFORMAT.D3DFMT_A8R8G8B8, _D3DPOOL.D3DPOOL_SYSTEMMEM, out var surf, IntPtr.Zero).ThrowOnError();
            using (var surface = new ComObject<IDirect3DSurface9>(surf))
            {
                var rc = new _D3DLOCKED_RECT();
                var rect = new tagRECT();
                surf.LockRect(ref rc, ref rect, 0).ThrowOnError();
                var pitch = rc.Pitch;
                surf.UnlockRect();

                var shots = new byte[count][];
                for (var i = 0; i < count; i++)
                {
                    shots[i] = new byte[pitch * mode.Height];
                }

                var sw = new Stopwatch();
                sw.Start();
                for (var i = 0; i < count; i++)
                {
                    dev.GetFrontBufferData(0, surf).ThrowOnError();
                    surf.LockRect(ref rc, ref rect, 0).ThrowOnError();
                    Marshal.Copy(rc.pBits, shots[i], 0, shots[i].Length);
                    surf.UnlockRect().ThrowOnError();
                }
                Console.WriteLine("Elapsed: " + sw.Elapsed);

                for (var i = 0; i < count; i++)
                {
                    SavePixelsToFile32bppPBGRA(mode.Width, mode.Height, (uint)pitch, shots[i], "cap" + i + ".png", WICConstants.GUID_ContainerFormatPng).ThrowOnError();
                }
            }
        }
    }
    return 0;
}

static HRESULT SavePixelsToFile32bppPBGRA(uint width, uint height, uint stride, byte[] pixels, string filePath, Guid format)
{
    if (filePath == null)
        throw new ArgumentNullException(nameof(filePath));

    using (var fac = new ComObject<IWICImagingFactory>((IWICImagingFactory)new WicImagingFactory()))
    {
        fac.Object.CreateStream(out var stream).ThrowOnError();
        using (new ComObject<IWICStream>(stream))
        {
            const int GENERIC_WRITE = 0x40000000;
            stream.InitializeFromFilename(filePath, GENERIC_WRITE).ThrowOnError();
            fac.Object.CreateEncoder(format, IntPtr.Zero, out var encoder).ThrowOnError();
            using (new ComObject<IWICBitmapEncoder>(encoder))
            {
                encoder.Initialize(stream, WICBitmapEncoderCacheOption.WICBitmapEncoderNoCache).ThrowOnError();
                encoder.CreateNewFrame(out var frame, out var bag).ThrowOnError();
                using (new ComObject<IWICBitmapFrameEncode>(frame))
                {
                    frame.Initialize(null).ThrowOnError();
                    frame.SetSize(width, height).ThrowOnError();
                    frame.SetPixelFormat(ref format).ThrowOnError();
                    frame.WritePixels(height, stride, stride * height, pixels).ThrowOnError();
                    frame.Commit().ThrowOnError();
                    encoder.Commit().ThrowOnError();
                }
            }
        }
    }
    return 0;
}

And the C++ version is this:

  HRESULT Direct3D9TakeScreenshots(UINT adapter, UINT count)
  {
    HRESULT hr = S_OK;
    IDirect3D9 *d3d = nullptr;
    IDirect3DDevice9 *device = nullptr;
    IDirect3DSurface9 *surface = nullptr;
    D3DPRESENT_PARAMETERS parameters = { 0 };
    D3DDISPLAYMODE mode;
    D3DLOCKED_RECT rc;
    UINT pitch;
    SYSTEMTIME st;
    LPBYTE *shots = nullptr;

    // init D3D and get screen size
    d3d = Direct3DCreate9(D3D_SDK_VERSION);
    HRCHECK(d3d->GetAdapterDisplayMode(adapter, &mode));

    parameters.Windowed = TRUE;
    parameters.BackBufferCount = 1;
    parameters.BackBufferHeight = mode.Height;
    parameters.BackBufferWidth = mode.Width;
    parameters.SwapEffect = D3DSWAPEFFECT_DISCARD;
    parameters.hDeviceWindow = NULL;

    // create device & capture surface
    HRCHECK(d3d->CreateDevice(adapter, D3DDEVTYPE_HAL, NULL, D3DCREATE_SOFTWARE_VERTEXPROCESSING, &parameters, &device));
    HRCHECK(device->CreateOffscreenPlainSurface(mode.Width, mode.Height, D3DFMT_A8R8G8B8, D3DPOOL_SYSTEMMEM, &surface, nullptr));

    // compute the required buffer size
    HRCHECK(surface->LockRect(&rc, NULL, 0));
    pitch = rc.Pitch;
    HRCHECK(surface->UnlockRect());

    // allocate screenshots buffers
    shots = new LPBYTE[count];
    for (UINT i = 0; i < count; i++)
    {
      shots[i] = new BYTE[pitch * mode.Height];
    }

    GetSystemTime(&st); // measure the time we spend doing <count> captures
    wprintf(L"%i:%i:%i.%i\n", st.wHour, st.wMinute, st.wSecond, st.wMilliseconds);
    for (UINT i = 0; i < count; i++)
    {
      // get the data
      HRCHECK(device->GetFrontBufferData(0, surface));

      // copy it into our buffers
      HRCHECK(surface->LockRect(&rc, NULL, 0));
      CopyMemory(shots[i], rc.pBits, rc.Pitch * mode.Height);
      HRCHECK(surface->UnlockRect());
    }
    GetSystemTime(&st);
    wprintf(L"%i:%i:%i.%i\n", st.wHour, st.wMinute, st.wSecond, st.wMilliseconds);

    // save all screenshots
    for (UINT i = 0; i < count; i++)
    {
      WCHAR file[100];
      wsprintf(file, L"cap%i.png", i);
      HRCHECK(SavePixelsToFile32bppPBGRA(mode.Width, mode.Height, pitch, shots[i], file, GUID_ContainerFormatPng));
    }

  cleanup:
    if (shots != nullptr)
    {
      for (UINT i = 0; i < count; i++)
      {
        delete shots[i];
      }
      delete[] shots;
    }
    RELEASE(surface);
    RELEASE(device);
    RELEASE(d3d);
    return hr;
  }

   HRESULT SavePixelsToFile32bppPBGRA(UINT width, UINT height, UINT stride, LPBYTE pixels, LPWSTR filePath, const GUID &format)
  {
    if (!filePath || !pixels)
      return E_INVALIDARG;

    HRESULT hr = S_OK;
    IWICImagingFactory *factory = nullptr;
    IWICBitmapEncoder *encoder = nullptr;
    IWICBitmapFrameEncode *frame = nullptr;
    IWICStream *stream = nullptr;
    GUID pf = GUID_WICPixelFormat32bppPBGRA;
    BOOL coInit = CoInitialize(nullptr);

    HRCHECK(CoCreateInstance(CLSID_WICImagingFactory, nullptr, CLSCTX_INPROC_SERVER, IID_PPV_ARGS(&factory)));
    HRCHECK(factory->CreateStream(&stream));
    HRCHECK(stream->InitializeFromFilename(filePath, GENERIC_WRITE));
    HRCHECK(factory->CreateEncoder(format, nullptr, &encoder));
    HRCHECK(encoder->Initialize(stream, WICBitmapEncoderNoCache));
    HRCHECK(encoder->CreateNewFrame(&frame, nullptr)); // we don't use options here
    HRCHECK(frame->Initialize(nullptr)); // we dont' use any options here
    HRCHECK(frame->SetSize(width, height));
    HRCHECK(frame->SetPixelFormat(&pf));
    HRCHECK(frame->WritePixels(height, stride, stride * height, pixels));
    HRCHECK(frame->Commit());
    HRCHECK(encoder->Commit());

  cleanup:
    RELEASE(stream);
    RELEASE(frame);
    RELEASE(encoder);
    RELEASE(factory);
    if (coInit) CoUninitialize();
    return hr;
  }