Export converted video into stream using ffmpeg pipe option

Question

I'm stream and successfully output converted video into mkv file.

var stream = ..incoming stream of type MemoryStream();
var process = new System.Diagnostics.Process();
process.StartInfo.Arguments = "-f mp4 -i pipe:0 -c:v libx264 -crf 20 -s 600:400 -f matroska myfile.mkv";
process.Start();
process.StandardInput.BaseStream.Write(stream.ToArray())
process.BeginOutputReadLine()
process.StandardInput.BaseStream.Close();
process.WaitForExit(1000);

My question is: how can I change the implementation (command and StandardOutput.BaseStream in order to export results into memory stream and not file (like it's in the above example).