Textbox and Delegate C#

textbox_delegate

        public delegate void delegate_write_output(String s);


        private void write_output(string str)
        {
            if (txt_output.InvokeRequired)
            {
                txt_output.Invoke(new delegate_write_output(write_output), new object[] { str });

            }
            else
            {
                txt_output.AppendText(str + "\r\n");
            }
        }