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PART I PART I PART I
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The C# Language
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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
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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 handlers in the X and Y classes, Handler( ), also have 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
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Use EventHandler
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For many events, the EventArgs parameter is unused To help facilitate the creation of code in these situations, the NET Framework includes a built-in delegate type called EventHandler, which can be used to declare event handlers in which no extra information is needed Here is an example that uses EventHandler:
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// 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 fire SomeEvent public void OnSomeEvent() { if(SomeEvent != null) SomeEvent(this, EventArgsEmpty); } } class EventDemo7 { static void Handler(object source, EventArgs arg) { ConsoleWriteLine("Event occurred"); ConsoleWriteLine("Source is " + source); }
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Delegates, Events, and Lambda Expressions
static void Main() { MyEvent evt = new MyEvent();
PART I PART I PART I
// Add Handler() to the event list evtSomeEvent += Handler; // Fire the event evtOnSomeEvent(); } }
In this case, the EventArgs parameter is unused and is passed the placeholder object EventArgsEmpty The output is shown here:
Event occurred Source is MyEvent
Applying Events: A Case Study
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 fired 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; } // Declare a delegate type for an event delegate void KeyHandler(object source, KeyEventArgs arg); // Declare a keypress event class class KeyEvent { public event KeyHandler KeyPress; // This is called when a key is pressed public void OnKeyPress(char key) {
Part I:
The C# Language
KeyEventArgs k = new KeyEventArgs(); if(KeyPress != null) { kch = key; KeyPress(this, k); } } } // Demonstrate KeyEvent class KeyEventDemo { static void Main() { KeyEvent kevt = new KeyEvent(); ConsoleKeyInfo key; int count = 0; // Use a lambda expression to display the keypress kevtKeyPress += (source, arg) => ConsoleWriteLine(" Received keystroke: " + argch); // Use a lambda expression to count keypresses kevtKeyPress += (source, arg) => count++; // count is an outer variable ConsoleWriteLine("Enter some characters " + "Enter a period to stop"); do { key = ConsoleReadKey(); kevtOnKeyPress(keyKeyChar); } while(keyKeyChar != ''); ConsoleWriteLine(count + " keys pressed"); } }
Here is a sample run:
Enter some characters Enter a period to stop t Received keystroke: t e Received keystroke: e s Received keystroke: s t Received keystroke: t Received keystroke: 5 keys pressed
The program begins by deriving a class from EventArgs called KeyEventArgs, which is used to pass a keystroke to an event handler Next, a delegate called KeyHandler defines the event handler for keystroke events The class KeyEvent encapsulates the keypress event It defines the event KeyPress In Main( ), a KeyEvent object called kevt is created Next, an event handler based on a lambda expression is added to kvetKeyPress that displays each key as it is entered, as shown here:
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