Method File.AppendAllText

     To add any text to test file, can be used method

File.AppendAllText(String path, String contents)

   1: string fileName = Path.GetTempFileName();

   2: string fileContent = "sample of method File.WriteAllText";

   3: File.WriteAllText(fileName, fileContent);

Additional Links:

File..::.AppendAllText Method (String, String)

File..::.AppendAllText Method (String, String, Encoding)