static void Main(string[] args)
{
var text = File.ReadAllText(@"C:words.txt");
File.WriteAllText(@"C:words.txt", text + "DERP");
}
using (var fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (var sr = new StreamReader(fs, Encoding.Default)) {
// read the stream
//...
}