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How to: Declare Events That Avoid Blocking

There are several circumstances when it is important that one event handler not block subsequent event handlers. Custom events allow the event to call its event handlers asynchronously.

By default, the backing-store field for an event declaration is a multicast delegate that serially combines all the event handlers. This means that if one handler takes a long time to complete, it blocks the other handlers until it completes. (Well-behaved event handlers should never perform lengthy or potentially blocking operations.)

Instead of using the default implementation of events that Visual Basic provides, you can use a custom event to execute the event handlers asynchronously.

In this example, the AddHandler accessor adds the delegate for each handler of the Click event to an ArrayList stored in the EventHandlerList field.

When code raises the Click event, the RaiseEvent accessor invokes all the event handler delegates asynchronously using the BeginInvoke method. That method invokes each handler on a worker thread and returns immediately, so handlers cannot block one another.

Public NotInheritable Class ReliabilityOptimizedControl
    'Defines a list for storing the delegates 
    Private EventHandlerList As New ArrayList

    'Defines the Click event using the custom event syntax. 
    'The RaiseEvent always invokes the delegates asynchronously 
    Public Custom Event Click As EventHandler
        AddHandler(ByVal value As EventHandler)
            EventHandlerList.Add(value)
        End AddHandler 
        RemoveHandler(ByVal value As EventHandler)
            EventHandlerList.Remove(value)
        End RemoveHandler 
        RaiseEvent(ByVal sender As Object, ByVal e As EventArgs)
            For Each handler As EventHandler In EventHandlerList
                If handler IsNot Nothing Then
                    handler.BeginInvoke(sender, e, Nothing, Nothing)
                End If 
            Next 
        End RaiseEvent 
    End Event 
End Class
Public NotInheritable Class ReliabilityOptimizedControl
    'Defines a list for storing the delegates 
    Private EventHandlerList As New ArrayList

    'Defines the Click event using the custom event syntax. 
    'The RaiseEvent always invokes the delegates asynchronously 
    Public Custom Event Click As EventHandler
        AddHandler(ByVal value As EventHandler)
            EventHandlerList.Add(value)
        End AddHandler 
        RemoveHandler(ByVal value As EventHandler)
            EventHandlerList.Remove(value)
        End RemoveHandler 
        RaiseEvent(ByVal sender As Object, ByVal e As EventArgs)
            For Each handler As EventHandler In EventHandlerList
                If handler IsNot Nothing Then
                    handler.BeginInvoke(sender, e, Nothing, Nothing)
                End If 
            Next 
        End RaiseEvent 
    End Event 
End Class

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