Programming Visual Basic .NET

Control Events

Controls on a form are represented in code as fields—one field for each control. For example, when the Visual Studio .NET Windows Forms Designer is used to add a text box to a form, the following declaration is added to the form class:

Private WithEvents TextBox1 As System.Windows.Forms.TextBox

This declaration doesn’t instantiate the control; it only defines a field that can hold a reference to a control of type TextBox. The control is instantiated in the InitializeComponent subroutine, which is called in the Form class’s constructor. The code that instantiates the control looks like this:

Me.TextBox1 = New System.Windows.Forms.TextBox(  )

As discussed in Chapter 2, when a field declaration includes the WithEvents keyword, the parent class can handle events that the referenced object raises. To do so, the parent class must define a handler method having the appropriate signature, and the definition of the method must include a Handles clause to link the method to the appropriate event on the appropriate object. For example, here is the definition of a handler method for the Click event of TextBox1:

Private Sub TextBox1_Click( _
   ByVal sender As Object, _
   ByVal e As System.EventArgs _
) Handles TextBox1.Click
   ' ...
End Sub

The event-handler method can be given any name, but it is a common convention to use a name of the form FieldName _ EventName. The event-handler method’s signature must correspond to the signature of the event being handled. By convention, event signatures ...