private void TextBox_KeyPress(object sender, KeyPressEventArgs e)
        {
            //only allow numbers and control keys
            if (!char.IsDigit(e.KeyChar) && !char.IsControl(e.KeyChar))  
            {
                e.Handled = true;
            }
            else if (char.IsControl(e.KeyChar))
            {
                e.Handled = false;
            }
            else
            {
                //using this rather than TextBox.Text.Length ensures
                //correct behaviour if user selects and overwrites contents of textbox
                int len = TextBox.Text.Length - TextBox.SelectionLength;
 
                //default to true then change to false if valid value entered
                e.Handled = true;
                switch (len)
                {
 
                    //allow 1 or 2 as first character
                    case 0:
                        if (e.KeyChar == '1' || e.KeyChar == '2')
                            e.Handled = false;
                        break;
 
                    //if this is the second character entered
                    case 1:
 
                        //check where user is typing. If user entered a number then moved the cursor back to start
                        //of textbox then still only allow a 1 or 2.
                        if (TextBox.SelectionStart == 0)
                        {
                            if (e.KeyChar >= '1' && e.KeyChar <= '2')
                                e.Handled = false;
                        }
                        //if the user is entering the second digit then allow 0 to 5
                        else
                        {
                            if (e.KeyChar >= '0' && e.KeyChar <= '5')
                                e.Handled = false;
                        }
                        break;
 
                    default:
                        break;
                }
            }
        }