Custom controls are a key theme in .NET development. They can help your programming style by improving encapsulation, simplifying a programming model, and making user interface more “pluggable”.

Developers often make a distinction between three or four types of controls:

User controls are the simplest type of control. They inherit from the System.Windows.Forms.UserControl class, and follow a model of composition. Usually, user controls combine more than one control in a logical unit (like a group of text boxes for entering address information).

Inherited controls are generally more powerful and flexible. With an inherited control, you choose the existing .NET control that is closest to what you want to provide. Then, you derive a custom class that overrides or adds properties and methods.

Owner-drawn controls generally use GDI+ drawing routines to generate their interfaces from scratch. Because of this, they tend to inherit from a base class like System.Windows.Forms.Control. Owner-drawn controls require the most work and provide the most customizable user interface.

Additionally, in this chapter you consider extender providers, which aren’t necessarily controls at all. These components add features to other controls on a form, and provide a remarkable way to implement extensible user interface.

The distinction above is slightly exaggerated. For example, you can create a user control that uses GDI+ drawing with other contained controls. Similarly, instead of inheriting from Control, UserControl, or a full-fledged .NET class, you can inherit from one of the intermediary classes to get a different level of support. For example, a control that contains other controls but handles its own output could inherit from ContainerControl, while a control that needs to provide scrolling might inherit from ScrollableControl.