How to: Declare Custom Events To Avoid Blocking (Visual Basic)
Updated: July 20, 2015
For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.
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
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