Chapter 12: Custom Controls

Overview

Visual Basic .NET provides a rich assortment of controls that you can use to build applications. However, those controls may not always be able to do what you need. In that case, you may want to build a control of your own. Building your own control lets you get exactly the behavior and appearance that you want.

Custom controls solve a couple of problems. First, they let you package a particular behavior or appearance so that you can easily reuse it later. If you need to draw one engineering diagram, you can draw it on a PictureBox. If you will need to draw many engineering diagrams (possibly in different applications), it would be easier to make an EngineeringDiagram control that you can use to make all the diagrams.

Another benefit to custom controls is that developers are familiar with controls and comfortable using them. Any experienced Visual Basic developer understands how to create instances of a control, set its properties, call its methods, and respond to its events. If you build a custom control to perform some complex task, developers already know a lot about how to use it. You just need to explain the specific features of your control.

Finally, controls can save and restore property information at design time. A developer can set properties for a control at design time, and the control uses those properties at runtime. This is useful for graphical controls, where properties such as Text, BackColor, and BorderStyle determine the controls’ appearance. It is also useful for nongraphical controls such as database connection, data adapter, DataSet, and DataView controls that use properties to determine what data is loaded and how it is arranged.

This chapter explains how to build custom controls. There are three main approaches to building custom controls. First, you can derive a control from an existing control. If a control already does most of what you want your custom control to do, you may be able to inherit from the existing control and avoid reimplementing all of its useful features.

The second way to build a custom control is to compose your control out of existing controls. For example, you might want to make a color selection control that enables the user to select red, green, and blue color components by using scroll bars, and then displays a sample of the resulting color. You could build this control using three scroll bars and a PictureBox. This gives you the advantages provided by the constituent controls without requiring you to re-implement their functionality.

Finally, you can build a custom control from scratch. This is the most work but gives you absolute control over everything that the control does.

This chapter explains the basics of building a control library project and testing its controls. It also describes the three approaches to building custom controls: deriving from an existing control, composing existing controls, and building a control from scratch.

A component is similar to a control that is invisible at runtime. Like controls, you can place components on a form at design time. Unlike controls, however, components do not sit on the form itself. Instead, they sit in the component tray below the form at design time, and they are invisible to the user at runtime. Most of this chapter’s discussion of custom controls applies equally to custom components.