qr code c# asp.net Delegates, Events, and Lambda Expressions in C#.NET

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Delegates, Events, and Lambda Expressions
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Notice that the handler is added using the += operator Events support only += and = In this case, Handler( ) is a static method, but event handlers can also be instance methods Finally, the event is raised as shown here:
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// Raise the event evtOnSomeEvent();
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PART I
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Calling OnSomeEvent( ) causes all registered event handlers to be called In this case, there is only one registered handler, but there could be more, as the next section explains
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A Multicast Event Example
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Like delegates, events can be multicast This enables multiple objects to respond to an event notification Here is an event multicast example:
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// An event multicast demonstration using System; // Declare a delegate type for an event delegate void MyEventHandler(); // Declare a class that contains an event class MyEvent { public event MyEventHandler SomeEvent; // This is called to raise the event public void OnSomeEvent() { if(SomeEvent != null) SomeEvent(); } } class X { public void Xhandler() { ConsoleWriteLine("Event received by X object"); } } class Y { public void Yhandler() { ConsoleWriteLine("Event received by Y object"); } } class EventDemo2 { static void Handler() { ConsoleWriteLine("Event received by EventDemo"); } static void Main() { MyEvent evt = new MyEvent(); X xOb = new X(); Y yOb = new Y();
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Part I:
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// Add handlers to the event list evtSomeEvent += Handler; evtSomeEvent += xObXhandler; evtSomeEvent += yObYhandler; // Raise the event evtOnSomeEvent(); ConsoleWriteLine(); // Remove a handler evtSomeEvent -= xObXhandler; evtOnSomeEvent(); } }
The output from the program is shown here:
Event received by EventDemo Event received by X object Event received by Y object Event received by EventDemo Event received by Y object
This example creates two additional classes, called X and Y, which also define event handlers compatible with MyEventHandler Thus, these handlers can also become part of the event chain Notice that the handlers in X and Y are not static This means that objects of each must be created, and the handler linked to each instance must be added to the event chain The differences between instance and static handlers are examined in the next section
Instance Methods vs Static Methods as Event Handlers
Although both instance methods and static methods can be used as event handlers, they do differ in one important way When a static method is used as a handler, an event notification applies to the class When an instance method is used as an event handler, events are sent to specific object instances Thus, each object of a class that wants to receive an event notification must register individually In practice, most event handlers are instance methods, but, of course, this is subject to the specific application Let s look at an example of each The following program creates a class called X that defines an instance method as an event handler This means that each X object must register individually to receive events To demonstrate this fact, the program multicasts an event to three objects of type X
/* Individual objects receive notifications when instance event handlers are used */ using System; // Declare a delegate type for an event delegate void MyEventHandler(); // Declare a class that contains an event class MyEvent {
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Delegates, Events, and Lambda Expressions
public event MyEventHandler SomeEvent; // This is called to raise the event public void OnSomeEvent() { if(SomeEvent != null) SomeEvent(); } } class X { int id; public X(int x) { id = x; } // This is an instance method that will be used as an event handler public void Xhandler() { ConsoleWriteLine("Event received by object " + id); } } class EventDemo3 { static void Main() { MyEvent evt = new MyEvent(); X o1 = new X(1); X o2 = new X(2); X o3 = new X(3); evtSomeEvent += o1Xhandler; evtSomeEvent += o2Xhandler; evtSomeEvent += o3Xhandler; // Raise the event evtOnSomeEvent(); } }
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