Programming Visual Basic .NET

Handling Form Events

The base Form class may at times raise events. These events can be handled by the derived Form class. One way to do this is to define a handler subroutine that uses the MyBase keyword in the Handles clause, like this:

' This is not the preferred technique.
Private Sub Form_Closing( _
   ByVal sender As Object, _
   ByVal e As System.ComponentModel.CancelEventArgs _
) Handles MyBase.Closing
   ' ...
End Sub

However, a better technique is to override the protected methods, which are provided by the Form class for this purpose. For example, the following method could be placed in the derived class’s definition, providing a way to respond to the form’s imminent closing:

' Assumes Imports System.ComponentModel
Protected Overrides Sub OnClosing( _
   ByVal e As CancelEventArgs _
)
   ' ...
   MyBase.OnClosing(e) ' Important
End Sub

Note that the implementation of the OnClosing method includes a call to the base class’s implementation. This is important. If this is not done, the Closing event won’t be raised, which will affect the behavior of any other code that has registered for the event.

Following is the list of events the Form class defines, including a brief description of each event and the syntax for overriding the protected method that corresponds to each event. Note also that the Form class indirectly derives from the Control class and that the Control class also exposes events and overridable methods that aren’t shown here.

Activated

Fired when the form is activated. Its syntax is:

Protected Overrides Sub OnActivated(ByVal e As System.EventArgs)

Closed

Fired when the form has been closed. Its syntax is:

Protected Overrides Sub OnClosed(ByVal e As System.EventArgs)

Closing

Fired when the form is about to close. Its syntax is:

Protected Overrides Sub OnClosing( _
   ByVal e As System.ComponentModel.CancelEventArgs)

The CancelEventArgs.Cancel property can be set to True to prevent the form from closing; its default value is False.

Deactivate

Fired when the form is deactivated. Its syntax is:

Protected Overrides Sub OnDeactivate(ByVal e As System.EventArgs)

InputLanguageChanged

Fired when the form’s input language has been changed. Its syntax is:

Protected Overrides Sub OnInputLanguageChanged( _
   ByVal e As System.Windows.Forms.InputLanguageChangedEventArgs)

The InputLanguageChangedEventArgs class has three properties that identify the new language: CharSet, which defines the character set associated with the new input language; Culture, which contains the culture code (see Appendix C) of the new input language; and InputLanguage, which contains a value indicating the new language.

InputLanguageChanging

Fired when the form’s input language is about to be changed. Its syntax is:

Protected Overrides Sub OnInputLanguageChanging( _
   ByVal e As System.Windows.Forms.InputLanguageChangingEventArgs)

The InputLanguageChangingEventArgs class has a Culture property that identifies the proposed new language and locale. It also has a Cancel property that can be set to True within the event handler to cancel the change of input language; the default value of the Cancel property is False.

Load

Fired when the form is loaded. Its syntax is:

Protected Overrides Sub OnLoad(ByVal e As System.EventArgs)

MaximizedBoundsChanged

Fired when the value of the form’s MaximizedBounds property (which determines the size of the maximized form) is changed. Its syntax is:

Protected Overrides Sub OnMaximizedBoundsChanged( _
   ByVal e As System.EventArgs)

MaximumSizeChanged

Fired when the value of the form’s MaximumSize property (which defines the maximum size to which the form can be resized) is changed. Its syntax is:

Protected Overrides Sub OnMaximumSizeChanged(ByVal e As System.EventArgs)

MdiChildActivate

Fired when an MDI child window is activated. Its syntax is:

Protected Overrides Sub OnMdiChildActivate(ByVal e As System.EventArgs)

MenuComplete

Fired when menu selection is finished. Its syntax is:

Protected Overrides Sub OnMenuComplete(ByVal e As System.EventArgs)

MenuStart

Fired when a menu is displayed. Its syntax is:

Protected Overrides Sub OnMenuStart(ByVal e As System.EventArgs)

MinimumSizeChanged

Fired when the value of the form’s MinimumSize property (which defines the minimum size to which the form can be resized) is changed. Its syntax is:

Protected Overrides Sub OnMinimumSizeChanged(ByVal e As System.EventArgs)