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PART I PART I PART I
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Event received by object 1 Event received by object 2 Event received by object 3
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As the output shows, each object registers its interest in an event separately, and each receives a separate notification Alternatively, when a static method is used as an event handler, events are handled independently of any object, as the following program shows:
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/* A class receives the notification when a static method is used as an event handler */ using System; // Declare a delegate type for an event
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Part I:
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delegate void MyEventHandler(); // Declare a class that contains an event class MyEvent { public event MyEventHandler SomeEvent; // This is called to fire the event public void OnSomeEvent() { if(SomeEvent != null) SomeEvent(); } } class X { /* This is a static method that will be used as an event handler */ public static void Xhandler() { ConsoleWriteLine("Event received by class"); } } class EventDemo4 { static void Main() { MyEvent evt = new MyEvent(); evtSomeEvent += XXhandler; // Fire the event evtOnSomeEvent(); } }
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Event received by class
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In the program, notice that no object of type X is ever created However, since Xhandler( ) is a static method of X, it can be attached to SomeEvent and executed when OnSomeEvent( ) is called
Using Event Accessors
The form of event used in the preceding examples created events that automatically manage the event handler invocation list, including the adding and subtracting of event handlers to and from the list Thus, you did not need to implement any of the list management functionality yourself Because they manage the details for you, these types of events are by far the most commonly used It is possible, however, to provide the event handler list operations yourself, perhaps to implement some type of specialized event storage mechanism To take control of the event handler list, you will use an expanded form of the event statement, which allows the use of event accessors The accessors give you control over how the event handler list is implemented This form is shown here:
15:
Delegates, Events, and Lambda Expressions
event event-delegate event-name { add { // code to add an event to the chain } remove { // code to remove an event from the chain } } This form includes the two event accessors add and remove The add accessor is called when an event handler is added to the event chain, by using += The remove accessor is called when an event handler is removed from the chain, by using = When add or remove is called, it receives the handler to add or remove as a parameter As with other types of accessors, this parameter is called value By implementing add and remove, you can define a custom event-handler storage scheme For example, you could use an array, a stack, or a queue to store the handlers Here is an example that uses the accessor form of event It uses an array to hold the event handlers Because the array is only three elements long, only three event handlers can be held in the chain at any one time
// Create a custom means of managing the event invocation list using System; // Declare a delegate type for an event delegate void MyEventHandler(); // Declare a class that holds up to 3 events class MyEvent { MyEventHandler[] evnt = new MyEventHandler[3]; public event MyEventHandler SomeEvent { // Add an event to the list add { int i; for(i=0; i < 3; i++) if(evnt[i] == null) { evnt[i] = value; break; } if (i == 3) ConsoleWriteLine("Event list full"); } // Remove an event from the list remove { int i; for(i=0; i < 3; i++) if(evnt[i] == value) {
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