C#中怎么写呢？

在C#中，可以使用DirectShowLib库实现DirectShow虚拟摄像头的输出。具体实现步骤如下：

1. 引用DirectShowLib库。可以通过NuGet包管理器添加DirectShowLib库，或者手动下载并添加到项目中。

2. 创建FilterGraph对象和CaptureGraphBuilder2对象。FilterGraph对象用于创建过滤器图形，而CaptureGraphBuilder2对象用于创建捕获图形。

   IFilterGraph2 filterGraph = new FilterGraph() as IFilterGraph2;
   ICaptureGraphBuilder2 captureGraphBuilder = new CaptureGraphBuilder2() as ICaptureGraphBuilder2;

3. 使用CaptureGraphBuilder2对象创建DirectShow虚拟摄像头和Sample Grabber过滤器。

   // 创建虚拟摄像头过滤器
   IBaseFilter virtualCameraFilter = null;
   Guid clsid = new Guid("{860BB310-5D01-11D0-BD3B-00A0C911CE86}");
   Guid iid = typeof(IBaseFilter).GUID;
   int hr = Activator.CreateInstance(ref clsid, null, CLSCTX_INPROC_SERVER, ref iid, out virtualCameraFilter);
   
   // 创建Sample Grabber过滤器
   IBaseFilter sampleGrabberFilter = new SampleGrabber() as IBaseFilter;

4. 将虚拟摄像头和Sample Grabber过滤器添加到FilterGraph对象中，并连接它们。

   // 将虚拟摄像头过滤器添加到FilterGraph中
   filterGraph.AddFilter(virtualCameraFilter, "Virtual Camera");
   
   // 将Sample Grabber过滤器添加到FilterGraph中
   filterGraph.AddFilter(sampleGrabberFilter, "Sample Grabber");
   
   // 连接虚拟摄像头和Sample Grabber过滤器
   IPin virtualCameraOutputPin = DsFindPin.ByDirection(virtualCameraFilter, PinDirection.Output, 0);
   IPin sampleGrabberInputPin = DsFindPin.ByDirection(sampleGrabberFilter, PinDirection.Input, 0);
   filterGraph.Connect(virtualCameraOutputPin, sampleGrabberInputPin);

5. 配置Sample Grabber过滤器，设置它的回调函数来处理视频帧。

   ISampleGrabber sampleGrabber = sampleGrabberFilter as ISampleGrabber;
   sampleGrabber.SetCallback(new SampleGrabberCallback(), 1);
   sampleGrabber.SetMediaType(new AMMediaType() { majorType = MediaType.Video });

6. 使用CaptureGraphBuilder2对象将Sample Grabber过滤器输出的视频流添加到捕获图形中，并将捕获图形作为DirectShow虚拟摄像头的输出。

   // 将Sample Grabber过滤器输出的视频流添加到捕获图形中
   hr = captureGraphBuilder.SetFiltergraph(filterGraph);
   hr = captureGraphBuilder.RenderStream(PinCategory.Capture, MediaType.Video, virtualCameraOutputPin, null, sampleGrabberFilter);
   
   // 将捕获图形作为DirectShow虚拟摄像头的输出
   IAMStreamConfig streamConfig = captureGraphBuilder as IAMStreamConfig;
   AMMediaType mediaType = new AMMediaType() { majorType = MediaType.Video };
   hr = streamConfig.SetFormat(mediaType);
   Marshal.ReleaseComObject(streamConfig);
   Marshal.ReleaseComObject(virtualCameraOutputPin);

7. 最后，启动FilterGraph对象并运行DirectShow虚拟摄像头。

   IMediaControl mediaControl = filterGraph as IMediaControl;
   mediaControl.Run();

这样就完成了在C#中实现DirectShow虚拟摄像头的输出。需要注意的是，以上代码仅为示例代码，实际使用时可能需要根据具体情况进行修改和调整。