IExtenderProvider Interface

An IExtenderProvider is a component that provides extender properties to other components. Extender properties are properties that are not defined by the component itself, but are provided by another component. For example, the ToolTip control is an IExtenderProvider. When you add a ToolTip control to a Form, all other controls on the form have a ToolTip property added to their list of properties.

To be considered an IExtenderProvider, a component must implement the IExtenderProvider interface. This interface defines a single method, CanExtend. The CanExtend method is used by the designer to determine which components in a container should receive the extender properties.


How to implement a VB.NET IExtenderProvider

To implement a VB.NET IExtenderProvider, you must:

  1. Create a class that inherits from Component.
  2. Implement the IExtenderProvider interface.
  3. Implement the CanExtend method.
  4. Define the extender properties that you want to provide.
  5. Implement the Get and Set methods for each extender property.

The following example shows how to implement a simple IExtenderProvider that provides a HelpText property to other controls:

Public Class HelpLabel Implements System.ComponentModel.IExtenderProvider Private helpTexts As Hashtable Private activeControl As System.Windows.Forms.Control Public Sub New() helpTexts = New Hashtable() End Sub Public Function CanExtend(ByVal extender As Object) As Boolean Implements System.ComponentModel.IExtenderProvider.CanExtend Return TypeOf extender Is Control End Function Public Function GetHelpText(ByVal control As Control) As String Return helpTexts(control) End Function Public Sub SetHelpText(ByVal control As Control, ByVal value As String) helpTexts(control) = value End Sub Protected Overrides Sub Dispose(disposing As Boolean) MyBase.Dispose(disposing) If disposing Then If helpTexts IsNot Nothing Then helpTexts.Clear() End If End If End Sub End Class

To use the HelpLabel IExtenderProvider, you would add it to a form and then set the HelpText property of any other controls on the form. The HelpLabel would then display the help text for the active control.

For example, the following code shows how to use the HelpLabel IExtenderProvider to display help text for a TextBox control:

Dim helpLabel As New HelpLabel() Me.Controls.Add(helpLabel) Dim textBox As New TextBox() textBox.HelpText = "This is the help text for the text box." Me.Controls.Add(textBox)

When the user clicks on the text box, the HelpLabel will display the help text for the text box.

IExtenderProviders can be used to create a wide variety of custom functionality. For example, you could use an IExtenderProvider to provide a custom validation property to other controls, or to provide a custom context menu property to other controls.


An IExtenderProvider is an interface used to extend the properties of controls at design-time without subclassing them. You implement this interface to add custom properties to specific controls, allowing you to provide additional design-time functionality. For example, you can create an IExtenderProvider to add custom properties to TextBox controls in a Windows Forms application, making them accessible and modifiable in the Visual Studio designer.