c# 关于.txt文件的写入

private void ChatProcess()
        {
            if (AcceptedSocket.Connected)
            {
                while (flag)
                {
                    Byte[] ReceivedByte = new Byte[64];
                    AcceptedSocket.Receive(ReceivedByte, ReceivedByte.Length, 0);
                    ReceivedStr = System.Text.Encoding.BigEndianUnicode.GetString(ReceivedByte)+"\r\n";
                  
                }
            }
        }
这是一个socket接收数据的函数，ReceivedStr 是接收到的数据的内容，我想把每次接收到的数据都放到一个。txt文件中，麻烦大家问一下我应该怎么做，我 是这么写的StreamWriter swMyfile = new StreamWriter(fsMyfile);
                    swMyfile.WriteLine(ReceivedStr);                  
                    swMyfile.Flush();但是都会提示流不能写入的错误
fsMyfile是FileStream fsMyfile = new FileStream("myfile.txt", FileMode.Create, FileAccess.ReadWrite);