While you might visualize a Visual Studio project as a series of procedures that execute in a sequence, in reality, most programs are event driven—meaning the flow of execution is determined by external occurrences called events.
An event is a signal that informs an application that something important has occurred. For example, when a user clicks a control on a form, the form can raise a Click event and call a procedure that handles the event. Events also allow separate tasks to communicate. Say, for example, that your application performs a sort task separately from the main application. If a user cancels the sort, your application can send a cancel event instructing the sort process to stop.
This section describes the terms and concepts used with events in Visual Basic.
You declare events within classes, structures, modules, and interfaces using the Event keyword, as in the following example:
Event AnEvent(ByVal EventNumber As Integer)
An event is like a message announcing that something important has occurred. The act of broadcasting the message is called raising the event. In Visual Basic, you raise events with the RaiseEvent statement, as in the following example:
Events must be raised within the scope of the class, module, or structure where they are declared. For example, a derived class cannot raise events inherited from a base class.
Any object capable of raising an event is an event sender, also known as an event source. Forms, controls, and user-defined objects are examples of event senders.
Event handlers are procedures that are called when a corresponding event occurs. You can use any valid subroutine with a matching signature as an event handler. You cannot use a function as an event handler, however, because it cannot return a value to the event source.
Visual Basic uses a standard naming convention for event handlers that combines the name of the event sender, an underscore, and the name of the event. For example, the Click event of a button named button1 would be named Sub button1_Click.
We recommend that you use this naming convention when defining event handlers for your own events, but it is not required; you can use any valid subroutine name.
Before an event handler becomes usable, you must first associate it with an event by using either the Handles or AddHandler statement.
WithEvents and the Handles Clause
The WithEvents statement and Handles clause provide a declarative way of specifying event handlers. An event raised by an object declared with the WithEvents keyword can be handled by any procedure with a Handles statement for that event, as shown in the following example:
' Declare a WithEvents variable. Dim WithEvents EClass As New EventClass ' Call the method that raises the object's events. Sub TestEvents() EClass.RaiseEvents() End Sub ' Declare an event handler that handles multiple events. Sub EClass_EventHandler() Handles EClass.XEvent, EClass.YEvent MsgBox("Received Event.") End Sub Class EventClass Public Event XEvent() Public Event YEvent() ' RaiseEvents raises both events. Sub RaiseEvents() RaiseEvent XEvent() RaiseEvent YEvent() End Sub End Class
The WithEvents statement and the Handles clause are often the best choice for event handlers because the declarative syntax they use makes event handling easier to code, read and debug. However, be aware of the following limitations on the use of WithEvents variables:
- You cannot use a WithEvents variable as an object variable. That is, you cannot declare it as Object—you must specify the class name when you declare the variable.
- Because shared events are not tied to class instances, you cannot use WithEvents to declaratively handle shared events. Similarly, you cannot use WithEvents or Handles to handle events from a Structure. In both cases, you can use the AddHandler statement to handle those events.
- You cannot create arrays of WithEvents variables.
WithEvents variables allow a single event handler to handle one or more kind of event, or one or more event handlers to handle the same kind of event.
Although the Handles clause is the standard way of associating an event with an event handler, it is limited to associating events with event handlers at compile time.
In some cases, such as with events associated with forms or controls, Visual Basic automatically stubs out an empty event handler and associates it with an event. For example, when you double-click a command button on a form in design mode, Visual Basic creates an empty event handler and a WithEvents variable for the command button, as in the following code:
Friend WithEvents Button1 As System.Windows.Forms.Button Protected Sub Button1_Click() Handles Button1.Click End Sub
AddHandler and RemoveHandler
The AddHandler statement is similar to the Handles clause in that both allow you to specify an event handler. However, AddHandler, used with RemoveHandler, provides greater flexibility than the Handles clause, allowing you to dynamically add, remove, and change the event handler associated with an event. If you want to handle shared events or events from a structure, you must use AddHandler.
AddHandler takes two arguments: the name of an event from an event sender such as a control, and an expression that evaluates to a delegate. You do not need to explicitly specify the delegate class when using AddHandler, since the AddressOf statement always returns a reference to the delegate. The following example associates an event handler with an event raised by an object:
AddHandler Obj.XEvent, AddressOf Me.XEventHandler
RemoveHandler , which disconnects an event from an event handler, uses the same syntax as AddHandler. For example:
RemoveHandler Obj.XEvent, AddressOf Me.XEventHandler
In the following example, an event handler is associated with an event, and the event is raised. The event handler catches the event and displays a message.
Then the first event handler is removed and a different event handler is associated with the event. When the event is raised again, a different message is displayed.
Finally, the second event handler is removed and the event is raised for a third time. Because there is no longer an event handler associated with the event, no action is taken.
Module Module1 Sub Main() Dim c1 As New Class1 ' Associate an event handler with an event. AddHandler c1.AnEvent, AddressOf EventHandler1 ' Call a method to raise the event. c1.CauseTheEvent() ' Stop handling the event. RemoveHandler c1.AnEvent, AddressOf EventHandler1 ' Now associate a different event handler with the event. AddHandler c1.AnEvent, AddressOf EventHandler2 ' Call a method to raise the event. c1.CauseTheEvent() ' Stop handling the event. RemoveHandler c1.AnEvent, AddressOf EventHandler2 ' This event will not be handled. c1.CauseTheEvent() End Sub Sub EventHandler1() ' Handle the event. MsgBox("EventHandler1 caught event.") End Sub Sub EventHandler2() ' Handle the event. MsgBox("EventHandler2 caught event.") End Sub Public Class Class1 ' Declare an event. Public Event AnEvent() Sub CauseTheEvent() ' Raise an event. RaiseEvent AnEvent() End Sub End Class End Module