textbox property

        protected override void OnKeyPress(KeyPressEventArgs e)
        {
            if (e.KeyChar >= '0' && e.KeyChar <= '9')
            {
                // move all digits over to the left
                Shift(); 
                // make sure there is a prompt to replace
                int index = this.Text.LastIndexOf(PromptChar);
                if (index >= 0)
                {
                    // insert the character typed into the last prompt character of the Text string
                    this.Text = this.Text.Substring(0, index) + e.KeyChar + this.Text.Substring(index);
                }
           }
            base.OnKeyPress(e);
        }