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Framework Fundamentals
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Events
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Most projects are nonlinear. In Windows Forms applications, you might have to wait for a user to click a button or press a key, and then respond to that event. In server applications, you might have to wait for an incoming network request. These capabilities are provided by events in the .NET Framework, as described in the following sections.
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What Is an Event
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An event is a message sent by an object to signal the occurrence of an action. The action could be caused by user interaction, such as a mouse click, or it could be triggered by some other program logic. The object that raises the event is called the event sender. The object that captures the event and responds to it is called the event receiver. In event communication, the event sender class does not know which object or method will receive (handle) the events it raises. What is needed is an intermediary (or pointer-like mechanism) between the source and the receiver. The .NET Framework defines a special type (Delegate) that provides the functionality of a function pointer.
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What Is a Delegate
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A delegate is a class that can hold a reference to a method. Unlike other classes, a delegate class has a signature, and it can hold references only to methods that match its signature. A delegate is thus equivalent to a type-safe function pointer or a callback. While delegates have other uses, the discussion here focuses on the event-handling functionality of delegates. A delegate declaration is sufficient to define a delegate class. The declaration supplies the signature of the delegate, and the common language runtime provides the implementation. The following example shows an event delegate declaration:
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' VB Public Delegate Sub AlarmEventHandler(sender As Object, e As EventArgs) // C# public delegate void AlarmEventHandler(object sender, EventArgs e);
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The standard signature of an event handler delegate defines a method that does not return a value, whose first parameter is of type Object and refers to the instance that raises the event, and whose second parameter is derived from type EventArgs and holds the event data. If the event does not generate event data, the second parameter
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Lesson 3: Constructing Classes
is simply an instance of EventArgs. Otherwise, the second parameter is a custom type derived from EventArgs and supplies any fields or properties needed to hold the event data. EventHandler is a predefined delegate that specifically represents an event handler method for an event that does not generate data. If your event does generate data, you must supply your own custom event data type and either create a delegate where the type of the second parameter is your custom type, or you must use the generic EventHandler delegate class and substitute your custom type for the generic type parameter. To associate the event with the method that will handle the event, add an instance of the delegate to the event. The event handler is called whenever the event occurs, unless you remove the delegate.
How to Respond to an Event
You must do two things to respond to an event:
Create a method to respond to the event. The method must match the Delegate signature. Typically, this means it must return void and accept two parameters: an Object and an EventArgs (or a derived class). The following code demonstrates this:
' VB Public Sub Button1_Click(sender As Object, e As EventArgs) ' Method code End Sub // C# private void button1_Click(object sender, EventArgs e) { // Method code }
Add the event handler to indicate which method should receive events, as the following code demonstrates:
' VB AddHandler Me.Button1.Click, AddressOf Me.Button1_Click // C# this.button1.Click += new System.EventHandler(this.button1_Click);
NOTE .NET 2.0
The .NET Framework 2.0 includes a new generic version of the EventHandler type.
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Framework Fundamentals
When the event occurs, the method you specified will run.
How to Raise an Event
At a minimum, you must do three things to raise an event:
Create a delegate:
' VB Public Delegate Sub MyEventHandler(ByVal sender As Object, ByVal e As EventArgs) // C# public delegate void MyEventHandler(object sender, EventArgs e);
Create an event member:
' VB Public Event MyEvent As MyEventHandler // C# public event MyEventHandler MyEvent;
Invoke the delegate within a method when you need to raise the event, as the following code demonstrates:
' VB Dim e As EventArgs = New EventArgs RaiseEvent MyEvent(Me, e) // C# MyEventHandler handler = MyEvent; EventArgs e = new EventArgs(); if (handler != null) { // Invokes the delegates. handler(this, e); } // Note that C# checks to determine whether handler is null. // This is not necessary in Visual Basic
Additionally, you can derive a custom class from EventArgs if you need to pass information to the event handler.
NOTE Differences in raising events in Visual Basic and C#
Visual Basic and C# differ when raising events. In C#, you must check whether the event is null before calling it. In Visual Basic, you can omit that check.
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