Close a file effectly

I have a piece of code to write a string line to a text file.

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

The query has a size of 2000 or more. So in my case I have to open and close file 2000 times. I felt that it is foolish.