Introduction to Exception Handling

• In a language without exception handling

–When an exception occurs, control goes to the operating system, where a message is displayed and the program is terminated

• In a language with exception handling

–Programs are allowed to trap some exceptions, thereby providing the possibility of fixing the problem and continuing

Exception Handling in C++

• Added to C++ in 1990
• Design is based on that of CLU, Ada, and ML

C++ Exception Handlers

• Exception Handlers Form:

  try {

— code that is expected to raise an exception

}

catch (formal parameter) {

— handler code

}

…

catch (formal parameter) {

— handler code

}

Introduction to Event Handling

• An event is a notification that something specific has occurred, such as a mouse click on a graphical button
• The event handler is a segment of code that is executed in response to an event

Event Handling in C#

• Event handling in C# (and the other .NET languages) is similar to that in Java
• .NET has two approaches, Windows Forms and Windows Presentation Foundation—we cover only the former (which is the original approach)
• An application subclasses the Form predefined class (defined in System.Windows.Forms)
• There is no need to create a frame or panel in which to place the GUI components
• Label objects are used to place text in the window
• Radio buttons are objects of the RadioButton class
• Components are positioned by assigning a new Point object to the Location property of the component  private RadioButton plain = new RadioButton();

  plain.Location = new Point(100, 300);

  plain.Text = ″Plain″;

  controls.Add(plain);

• All C# event handlers have the same protocol, the return type is void and the two parameters are of types object and EventArgs
• An event handler can have any name
• A radio button is tested with the Boolean Checked property of the button

    private void rb_CheckedChanged (object o,

  EventArgs e) {

  if (plain.Checked) …

  …

  }

• To register an event, a new EventHandler object must be created and added to the predefined delegate for the event

• When a radio button changes from unchecked to checked, the CheckedChanged event is raised
• The associated delegate is referenced by the name of the event
• If the handler was named rb_CheckedChanged, we could register it on the radio button named plain with:

plain.CheckedChanged +=

new EventHandler (rb_CheckedChanged);