.NET Windows Forms in a Nutshell

Extender Providers

As we saw in the previous section, it is possible to extend the set of properties that a control presents at design time by writing a custom designer class and implementing one or more of the metadata filtering methods. But what if you want to extend the property set of more than one control? You could write a designer class that is used as the base class for several controls’ designers, but what if you would like to be able to extend the property sets of controls you did not write?

The Forms Designer supports a special kind of component called an extender provider, which is able to extend the property set of any control. For example, the built-in ToolTip component is an extender provider—if you add a ToolTip component to a form, every control on the form gets an extra ToolTip property. (Such properties are known as extender properties.) We already saw how to use extender providers in Chapter 3, but we will now see how to implement such a component.

Extender providers are not controls, because they do not participate directly in the user interface. In the Forms Designer, they appear in the component tray rather than on the form itself, as shown in Figure 9-6. So instead of inheriting from the Control class, an extender provider must derive directly from the Component class, which is defined in the System.ComponentModel namespace.

Figure 9-6. An extender provider ...