How to: Declare Custom Events To Conserve Memory (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 an application keep its memory usage low. Custom events allow the application to use memory only for the events that it handles.

By default, when a class declares an event, the compiler allocates memory for a field to store the event information. If a class has many unused events, they needlessly take up memory.

Instead of using the default implementation of events that Visual Basic provides, you can use custom events to manage the memory usage more carefully.

In this example, the class uses one instance of the EventHandlerList class, stored in the Events field, to store information about the events in use. The EventHandlerList class is an optimized list class designed to hold delegates.

All events in the class use the Events field to keep track of what methods are handling each event.

    Public Class MemoryOptimizedBaseControl
        ' Define a delegate store for all event handlers.
        Private Events As New System.ComponentModel.EventHandlerList

        ' Define the Click event to use the delegate store.
        Public Custom Event Click As EventHandler
            AddHandler(ByVal value As EventHandler)
                Events.AddHandler("ClickEvent", value)
            End AddHandler
            RemoveHandler(ByVal value As EventHandler)
                Events.RemoveHandler("ClickEvent", value)
            End RemoveHandler
            RaiseEvent(ByVal sender As Object, ByVal e As EventArgs)
                CType(Events("ClickEvent"), EventHandler).Invoke(sender, e)
            End RaiseEvent
        End Event

        ' Define the DoubleClick event to use the same delegate store.
        Public Custom Event DoubleClick As EventHandler
            AddHandler(ByVal value As EventHandler)
                Events.AddHandler("DoubleClickEvent", value)
            End AddHandler
            RemoveHandler(ByVal value As EventHandler)
                Events.RemoveHandler("DoubleClickEvent", value)
            End RemoveHandler
            RaiseEvent(ByVal sender As Object, ByVal e As EventArgs)
                CType(Events("DoubleClickEvent"), EventHandler).Invoke(sender, e)
            End RaiseEvent
        End Event

        ' Define additional events to use the same delegate store.
        ' ...
    End Class

EventHandlerList
Events
How to: Declare Custom Events To Avoid Blocking

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