Programming Visual Basic .NET

Creating a Control

A control is a component with a visual representation. The Windows Forms class library provides the base functionality for controls through the Control class (defined in the System.Windows.Forms namespace). All controls derive directly or indirectly from the Control class. In addition, Windows Forms provides a class called UserControl for the purpose of making it easy to write custom control classes. The derivation of the UserControl class is shown in Figure 5-27.

Figure 5-27. The derivation hierarchy of the UserControl class

Building Controls from Other Controls

The easiest way to create a new control is to aggregate and modify the functionality of one or more existing controls. To do this in Visual Studio .NET’s Windows Forms Designer, perform the following steps:

1. Choose Project→Add User Control from the main menu.

2. Type the name of the .vb file that will hold the code for the control, and click OK. The designer displays a blank user control in design mode, as shown in Figure 5-28.

   

   Figure 5-28. A blank user control in Visual Studio .NET’s Windows Forms Designer

3. Add controls from the Toolbox window just as you would when laying out a form. Controls that are made part of another control are called constituent controls . Figure 5-29 shows a user control that has two ...