private void bt_test_Click(object sender, EventArgs e)
        {
            IPAddress[] ipAddress =  Dns.GetHostAddresses("whois.verisign-grs.com");
            byte[] receiveBufferStr = new byte[1024];
            IPEndPoint ipep = new IPEndPoint(ipAddress[0],43);
            Socket socket = new Socket(ipep.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
            string result = "";
            int i = 0;
            
            
            socket.Connect(ipep);
            socket.Send(Encoding.ASCII.GetBytes("waihuiqq.com\r\n"));
            do
            {
                i = socket.Receive(receiveBufferStr);
                string temp  = Encoding.ASCII.GetString(receiveBufferStr);
                result += temp;//此处会都多次执行到，但result的值一直是第一次的值
                //rtb_result.AppendText(temp);//可以得到完整的结果
            } while (i > 0);
            rtb_result.Clear();
            rtb_result.AppendText(result);
            MessageBox.Show("完成");
        }

string temp = "";
do
{
      i = socket.Receive(receiveBufferStr);
      temp  = Encoding.ASCII.GetString(receiveBufferStr);
      result += temp;
} while (i > 0);

------解决方案--------------------

“得不到预期的效果”这个词儿太泛泛了，谁知道你得到的真实效果是什么呢？

receiveBufferStr中只有前i个字节才是接收到的数据，你为什么要把整个receiveBufferStr都进行解码呢？