Delegates and the AddressOf Operator

Artigo
11/18/2006

Delegates are objects you can use to call the methods of other objects. They are sometimes described as type-safe function pointers because they are similar to function pointers used in other programming languages. But unlike function pointers, Visual Basic delegates are a reference type based on the class System.Delegate. Delegates can reference both shared methods — methods that can be called without a specific instance of a class — and instance methods.

Delegates and Events

Delegates are useful in situations where you need an intermediary between a calling procedure and the procedure being called. For example, you might want an object that raises events to be able to call different event handlers under different circumstances. Unfortunately, the object raising the events cannot know ahead of time which event handler is handling a specific event. Visual Basic lets you dynamically associate event handlers with events by creating a delegate for you when you use the AddHandler statement. At run time, the delegate forwards calls to the appropriate event handler.

Although you can create your own delegates, in most cases Visual Basic creates the delegate and takes care of the details for you. For example, an Event statement implicitly defines a delegate class named <EventName>EventHandler as a nested class of the class containing the Event statement, and with the same signature as the event. The AddressOf statement implicitly creates an instance of a delegate. For example, the following two lines of code are equivalent:

AddHandler Button1.Click, AddressOf Me.Button1_Click
' The previous line of code is shorthand for the next line of code.
AddHandler Button1.Click, New EventHandler(AddressOf Button1_Click)

You can use the shorthand way of creating delegates anywhere the compiler can determine the delegate's type by the context.

Declaring Events that Use an Existing Delegate Type

In some situations, you may want to declare an event to use an existing delegate type as its underlying delegate. The following syntax demonstrates how:

Delegate Sub DelegateType()
Event AnEvent As DelegateType

This is useful when you want to route multiple events to the same handler.

Delegate Variables and Parameters

You can use delegates for other, non-event related tasks, such as free threading or with procedures that need to call different versions of functions at compile time.

For example, suppose you have a classified-ad application that includes a list box with the names of cars. The ads are sorted by title, which is normally the make of the car. A problem you may face occurs when some cars include the year of the car before the make. The problem is that the built-in sort functionality of the list box sorts only by character codes; it places all the ads starting with dates first, followed by the ads starting with the make.

To fix this, you can create a sort procedure in a class that uses the standard alphabetic sort on most list boxes, but is able to switch at run time to the custom sort procedure for car ads. To do this, you pass the custom sort procedure to the sort class at run time, using delegates.

Each delegate class defines a constructor that takes a specification of an object method. The following syntax provides an example of a parameter to such a delegate constructor:

AddressOf [expression.]methodname

The compile-time type of expression must be a class or an interface with a method by the specified name whose signature matches the signature of the delegate class. The method can be either a shared (class) method or an instance method.

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Delegates and the AddressOf Operator

Delegates and Events

Declaring Events that Use an Existing Delegate Type

Delegate Variables and Parameters

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