Step by Step: Event handling in VB.NET

Introduction

In this step by step article, we look at implementing event handling in VB.NET.

Implementing events

Step 1 - Create an EventArgs class

If you need to pass arguments to the event handler, a specific EventArgs class has to be made. Eventually, a suitable EventArgs class might already be available, but in most cases, you will have to create one to tailor your specific arguments.

If you do not need to pass arguments to the event handler (except the sender), then no subclass of EventArgs is needed. However, note that you should still define the event method with an argument of type System.EventArgs, and then pass System.EventArgs.Empty.

Create the EventArgs as follows:
```
Public Class StartEventArgs
    Inherits System.EventArgs

    'Provide one or more constructors, as well as fields and
    'accessors for the arguments.

End Class
```
Note: All EventArgs classes should have a name ending by EventArgs.

Step 2 - Create an event

For each kind of event, an Event is created in the sending class:

Public Class Sender

    Public Event Start(ByVal sender As Object, _ 
             ByVal e As StartEventArgs)

    '...

End Class

Note: In VB.NET no explicit declaration of the delegate class is needed.

Step 3 - Create OnEvent methods

Although not required, it is a good practice to create protected overridable OnEvent methods for your events:
```
Protected Overridable Sub OnStart(ByVal e As StartEventArgs)
    RaiseEvent Start(Me, e)
End Sub
```
You can now call this method whenever you want to send the Start event. i.e.:
```
OnStart(New StartEventArgs(data))
```
Step 4 - Make the event default

When your class represents a component, it is a good practice to make the most commonly used event, the default event. Add a DefaultEvent attribute in front of your sender class:
```
<System.ComponentModel.DefaultEvent("Start")> _
    Public Class Sender
    
    Inherits System.ComponentModel.Component
    '...
```

Handling events (static)

Under handling events statically, we understand having a WithEvents field to the sender.

Any field pointing to an object that can send events, can have the WithEvents modifier, in which case you can easily write event handlers by creating subroutines with the same signature as the event, and marked as Handles <sender>.<event>. The name of the method is of no matter, but by default, VS.NET uses <fieldname>_<event>:

Public Class Receiver

    Friend WithEvents MySender As Sender

    Private Sub MySender_Start(ByVal sender As Object, _
         ByVal e As StartEventArgs) Handles MySender.Start
    
        '...
    End Sub

End Class

Handling events (dynamic)

When you have no field to the event sending object, i.e. because the object is passed as parameter to a method that can be called an unspecified number of times, then you can dynamically register to the event using the AddHandler operation:

AddHandler sender.Start, AddressOf Me.HandleStart

To remove the handler, use the RemoveHandler operation:

RemoveHandler sender.Start, AddressOf Me.HandleStart

The handling method is a regular subroutine with the same signature as the event (no Handles is needed):

Public Sub HandleStart(ByVal sender As Object, _
           ByVal e As System.EventArgs)
    '...
End Sub

Code sample

Public Class StartEventArgs
    Inherits System.EventArgs

    'Provide one or more constructors, as well as fields and
    'accessors for the arguments.

End Class

Public Class Sender

    Public Event Start(ByVal sender As Object, ByVal e As StartEventArgs)

    Protected Overridable Sub OnStart(ByVal e As StartEventArgs)
        RaiseEvent Start(Me, e)
    End Sub

    '...

End Class

Public Class Receiver

    Friend WithEvents MySender As Sender

    Private Sub MySender_Start(ByVal sender As Object, _
          ByVal e As StartEventArgs) Handles MySender.Start
        '...
    End Sub

End Class