private void DispenseButton_Click( object sender, EventArgs e )
      {
         try
         {
            // get user input
            int selection = Convert.ToInt32( SelectionTextBox.Text );
            OutputLabel.Text = SnacksString[ selection ].ToString() +
               " has been dispensed";
         } // end try
         catch ( FormatException formatExceptionParameter )
         {
            // handle case when user enters invalid input
            MessageBox.Show( "Please enter an integer value.",
               "Number Format Exception", MessageBoxButtons.OK,
               MessageBoxIcon.Error );

            SelectionTextBox.Text = "";
            SelectionTextBox.Focus();
         } // end catch
         catch ( IndexOutOfRangeException indexExceptionParameter )
         {
            // handle case when user inputs number not in array bounds
            MessageBox.Show( "Please enter a value between 0-3.",
               "Array Index Out of Bounds Error", MessageBoxButtons.OK,
               MessageBoxIcon.Error );

            SelectionTextBox.Text = "";
            SelectionTextBox.Focus();
         } // end catch
      } // end method DispenseButton_Click