public class TCPCMD
    {
        public class CMD
        {
            public  string Data { get; set; }
        }
        public CMD cmd { get; set; } //这句必须加，否则反序列化会失败
    }

      TCPCMD tc = JsonConvert.DeserializeObject<TCPCMD>(str);
           
            Debug.WriteLine($"SetPoint {str}");

            debug.writeline(tc.cmd.Data);