PART I in Visual C#.NET

Generation QR Code in Visual C#.NET PART I

Part I:

Here is the output:

Event 0 received by an X object Source is MyEvent Event 0 received by a Y object Source is MyEvent Event 1 received by an X object Source is MyEvent Event 1 received by a Y object Source is MyEvent

In this example, MyEventArgs is derived from EventArgs MyEventArgs adds just one field of its own: EventNum The event handler delegate MyEventHandler now takes the two parameters required by the NET Framework As explained, the first is an object reference to the generator of the event The second is a reference to EventArgs or a class derived from EventArgs The event handler in the X and Y classes, Handler( ), also has the same types of parameters Inside MyEvent, a MyEventHandler called SomeEvent is declared In the OnSomeEvent( ) method, SomeEvent is called with the first argument being this, and the second argument being a MyEventArgs instance Thus, the proper arguments are passed to MyEventHandler to fulfill the requirements for NET compatibility

The previous program declared its own event delegate However, there is no need to do this because the NET Framework provides a built-in generic delegate called EventHandler<TEventArgs> (See 18 for a discussion of generic types) Here, the type of TEventArgs specifies the type of the argument passed to the EventArgs parameter of the event For example, in the preceding program, SomeEvent in MyEvent could have been declared like this:

public event EventHandler<MyEventArgs> SomeEvent;

In general, it is better to use this approach rather than defining your own delegate For many events, the EventArgs parameter is unused To help facilitate the creation of code in these situations, the NET Framework includes a non-generic delegate called EventHandler, which can be used to declare event handlers in which no extra information is needed Here is an example that uses EventHandler:

// Use the built-in EventHandler delegate using System; // Declare a class that contains an event class MyEvent { public event EventHandler SomeEvent; // uses EventHandler delegate // This is called to raise SomeEvent public void OnSomeEvent() {

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if(SomeEvent != null) SomeEvent(this, EventArgsEmpty); }

} class EventDemo7 { static void Handler(object sender, EventArgs e) { ConsoleWriteLine("Event occurred"); ConsoleWriteLine("Source is " + sender); } static void Main() { MyEvent evt = new MyEvent(); // Add Handler() to the event list evtSomeEvent += Handler; // Raise the event evtOnSomeEvent(); } }

In this case, the EventArgs parameter is unused and is passed the placeholder object EventArgsEmpty The output is shown here:

Events are frequently used in message-based environments such as Windows In such an environment, a program simply waits until it receives a message, and then it takes the appropriate action Such an architecture is well suited for C#-style event handling because it is possible to create event handlers for various messages and then simply invoke a handler when a message is received For example, the left-button mouse click message could be tied to an event called LButtonClick When a left-button click is received, a method called OnLButtonClick( ) can be called, and all registered handlers will be notified Although developing a Windows program that demonstrates this approach is beyond the scope of this chapter, it is possible to give an idea of how such an approach would work The following program creates an event handler that processes keystrokes The event is called KeyPress, and each time a key is pressed, the event is raised by calling OnKeyPress( ) Notice that NET-compatible events are created and that lambda expressions provide the event handlers

// A keypress event example using System; // Derive a custom EventArgs class that holds the key class KeyEventArgs : EventArgs { public char ch; }

Part I: