How can I clear the content of a file?

You can use the File.WriteAllText method.

System.IO.File.WriteAllText(@"Path/foo.bar",string.Empty);

This is what I did to clear the contents of the file without creating a new file as I didn't want the file to display new time of creation even when the application just updated its contents.

FileStream fileStream = File.Open(<path>, FileMode.Open);

/* 
 * Set the length of filestream to 0 and flush it to the physical file.
 *
 * Flushing the stream is important because this ensures that
 * the changes to the stream trickle down to the physical file.
 * 
 */
fileStream.SetLength(0);
fileStream.Close(); // This flushes the content, too.

Use FileMode.Truncate everytime you create the file. Also place the File.Create inside a try catch.