单声道编译cs代码出现此错误意外的符号SpeechSynthesizer？

Question

我可以解决这个错误

我使用此命令

mcs -out:hello.exe hello.cs

请帮帮我

using System;
using System.Speech.Synthesis;
      SpeechSynthesizer synth = new SpeechSynthesizer();
      synth.SetOutputToDefaultAudioDevice();
      synth.Speak("This example demonstrates a basic use of Speech Synthesizer");
      Console.WriteLine();
      Console.WriteLine("Press any key to exit...");
      Console.ReadKey();