How to both read and write a file in C#
You need a single stream, opened for both reading and writing.
FileStream fileStream = new FileStream(
@"c:\words.txt", FileMode.OpenOrCreate,
FileAccess.ReadWrite, FileShare.None);
Don't forget the easy route:
static void Main(string[] args)
{
var text = File.ReadAllText(@"C:\words.txt");
File.WriteAllText(@"C:\words.txt", text + "DERP");
}