C# 0 The Complete Reference

PART II

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857

Abor t

Cancel

Ignore

No

None

OK

Retr y

Yes

Your program can examine the return value to determine the course of action the user desires. 

For example, if the message box prompts the user before overwriting a file, your program can 

prevent the overwrite if the user clicks Cancel, and it can allow the overwrite if the user clicks OK.

The following program adds a stop button and a message box to the preceding example. 

In the stop button handler, a message box is displayed that asks the user if he or she wants 

to stop the program. If the user clicks Yes, the program is stopped. If the user clicks No, the 

program continues running.

// Add a stop button.

using System;

using System.Windows.Forms;

using System.Drawing;

class ButtonForm : Form {

  Button MyButton;

  Button StopButton;

  public ButtonForm() {

    Text = "Adding a Stop Button";

    // Create the buttons.

    MyButton = new Button();

    MyButton.Text = "Press Here";

    MyButton.Location = new Point(100, 200);

    StopButton = new Button();

    StopButton.Text = "Stop";

    StopButton.Location = new Point(100, 100);

    // Add the button event handlers to the window.

    MyButton.Click += MyButtonClick;

    Controls.Add(MyButton);

    StopButton.Click += StopButtonClick;

    Controls.Add(StopButton);

  }

  [STAThread]

  static void Main() {

    ButtonForm skel = new ButtonForm();

    Application.EnableVisualStyles();

    Application.Run(skel);

  }

  // Handler for MyButton.

  void MyButtonClick(object who, EventArgs e) {

    if(MyButton.Top == 200)

      MyButton.Location = new Point(10, 10);

    else

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      MyButton.Location = new Point(100, 200);

  }

  // Handler for StopButton.

  void StopButtonClick(object who, EventArgs e) {

    // If user answers Yes, terminate the program.

    DialogResult result = MessageBox.Show("Stop Program?",

                            "Terminate",

                            MessageBoxButtons.YesNo);

    if(result == DialogResult.Yes) Application.Exit();

  }

}

Let’s look closely at how the message box is used. Inside the 

ButtonForm

 constructor, a 

second button is added. This button contains the text “Stop,” and its event handler is linked 

to