C# - Read File into Textbox

        private void ReadFile(string filename)
        {
            int counter = 0;
            string line;
            System.IO.StreamReader file =
               new System.IO.StreamReader(filename);
            while ((line = file.ReadLine()) != null)
            {
                filetxt.AppendText(line + Environment.NewLine);
                counter++;
            }

            file.Close();
        }