如何用c#查找和替换文件中的文本

My code so far

到目前为止我的代码

StreamReader reading = File.OpenText("test.txt");
string str;
while ((str = reading.ReadLine())!=null)
{
      if (str.Contains("some text"))
      {
          StreamWriter write = new StreamWriter("test.txt");
      }
}
StreamReader readi