Close a file effectly

My case is a little complex.

 while (!hasHitEnd)
 {
   foreach (var y in query)
   {
      TextWriter tw = new StreamWriter(outputFileName, true);
      tw.WriteLine(y.Path + "_" + y.Name);
      tw.Close();
   }
 }

There is a while loop outside of foreach query, your File.WriteAllLines will create a new file. I just want to append line to the existing file.