Handles Clause (Visual Basic)
Updated: July 20, 2015
Declares that a procedure handles a specified event.
proceduredeclaration Handles eventlist
Sub procedure declaration for the procedure that will handle the event.
List of the events for
proceduredeclaration to handle, separated by commas. The events must be raised by either the base class for the current class, or by an object declared using the
Handles keyword at the end of a procedure declaration to cause it to handle events raised by an object variable declared using the
WithEvents keyword. The
Handles keyword can also be used in a derived class to handle events from a base class.
Handles keyword and the
AddHandler statement both allow you to specify that particular procedures handle particular events, but there are differences. Use the
Handles keyword when defining a procedure to specify that it handles a particular event. The
AddHandler statement connects procedures to events at run time. For more information, see AddHandler Statement.
For custom events, the application invokes the event's
AddHandler accessor when it adds the procedure as an event handler. For more information on custom events, see Event Statement.
Public Class ContainerClass ' Module or class level declaration. WithEvents Obj As New Class1 Public Class Class1 ' Declare an event. Public Event Ev_Event() Sub CauseSomeEvent() ' Raise an event. RaiseEvent Ev_Event() End Sub End Class Sub EventHandler() Handles Obj.Ev_Event ' Handle the event. MsgBox("EventHandler caught event.") End Sub ' Call the TestEvents procedure from an instance of the ContainerClass ' class to test the Ev_Event event and the event handler. Public Sub TestEvents() Obj.CauseSomeEvent() End Sub End Class
The following example demonstrates how a derived class can use the
Handles statement to handle an event from a base class.
Public Class BaseClass ' Declare an event. Event Ev1() End Class Class DerivedClass Inherits BaseClass Sub TestEvents() Handles MyBase.Ev1 ' Add code to handle this event. End Sub End Class
The following example contains two button event handlers for a WPF Application project.
Private Sub Button1_Click(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles Button1.Click MessageBox.Show(sender.Name & " clicked") End Sub Private Sub Button2_Click(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles Button2.Click MessageBox.Show(sender.Name & " clicked") End Sub
The following example is equivalent to the previous example. The
eventlist in the
Handles clause contains the events for both buttons.
Private Sub Button_Click(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles Button1.Click, Button2.Click MessageBox.Show(sender.Name & " clicked") End Sub