StreamWriter怎么循环追加到指定文件

StreamWriter如何循环追加到指定文件？
StreamReader sr = new StreamReader(@"C:\Users\xwy\Desktop\123.txt", Encoding.GetEncoding("GB2312"));
  StreamWriter sw = new StreamWriter(@"C:\Users\xwy\Desktop\T.txt", true);
  while (sr.EndOfStream)
  {
  string row = sr.ReadLine();
  string[] columns = row.Split(new char[] { ',' });
  sw.WriteLine("xxxxxx{0}xxxx{1}xxxx",columns [1],columns [2]);
  sw.Flush();
  sw.Close();
  }
先读取123.txt的每一行，然后用,分割成一块块，接着输出。。。请问这代码哪里错了，我只生成了文件，没写入内容

[解决办法]
代码如下，测试通过

C# code

 StreamReader sr = new StreamReader(@"C:\Users\xwy\Desktop\123.txt", Encoding.GetEncoding("GB2312"));            StreamWriter sw = new StreamWriter(@"C:\Users\xwy\Desktop\T.txt", true);            while (!sr.EndOfStream)            {                string row = sr.ReadLine();                string[] columns = row.Split(new char[] { ',' });                sw.WriteLine("xxxxxx{0}xxxx{1}xxxx", columns[0], columns[1]);            }            sw.Flush();//关闭不要放到循环里面            sw.Close();