如何重写该函数，使其返回想要的值

问题遇到的现象和发生背景

//新建一个服务端
var testPromise = new TaskCompletionSource();
            //var tlsCertificate = TestResourceHelper.GetTestCertificate();
            Func<Task> closeServerFunc = await this.StartServerAsync(true, ch =>
            {
                ch.Pipeline.AddLast("server logger", new LoggingHandler("SERVER"));
                ch.Pipeline.AddLast("server logger2", new LoggingHandler("SER***"));
                ch.Pipeline.AddLast("server_decoder", new JsonObjectDecoder());
                ch.Pipeline.AddLast(new EchoChannelHandler());
            }, testPromise, Port+1);

class EchoChannelHandler : ChannelHandlerAdapter
    {
        //收到消息后回复(现在是收到什么回复什么)
        public override void ChannelRead(IChannelHandlerContext context, object message) => context.Channel.WriteAsync(message);
   }

问题相关代码，请勿粘贴截图

运行结果及报错内容

我的解答思路和尝试过的方法

//我尝试重写这个方法，让他返回“abc”,但是这样写返回值是null
 public override void ChannelRead(IChannelHandlerContext context, object message) 
        {
             string echo = "abc";
            context.Channel.WriteAsync(echo);
       }

我想要达到的结果想要达到的结果

下面这样也不行？

        class EchoChannelHandler : ChannelHandlerAdapter
        {
            public override void ChannelRead(IChannelHandlerContext context, object message) => context.Channel.WriteAsync((object)"abc");
        }