Mastering Visual Basic .NET

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Chapter 9 BUILDING CUSTOM WINDOWS CONTROLS
already know how to add features like the ones described here to a TextBox from within the application’s
code. But what if you want to enhance multiple TextBox controls on the same form, or reuse
your code in multiple applications?
The best approach is to create a new Windows control with all the desired functionality and then
reuse it in multiple projects. To use the proper terminology, you can create a new custom Windows
control that inherits from the TextBox control. The derived (or subclassed) control includes all the
functionality of the control being inherited, plus any new features you care to add to it. This is
exactly what we’re going to do in the following section.
Building the FocusedTextBox Control
Let’s call our new custom control FocusedTextBox. Start a new VB project and, on the New Project
dialog box, select the template Windows Control Library. Name the project FocusedTextBox. The
Solution Explorer for this project contains a single item, the UserControl1 item (in addition to the
standard project components such as References and AssemblyInfo). UserControl1 (Figure 9.1) is
the control’s surface—in a way, it’s the control’s form. This is where you’ll design the visible interface
of the new control.
Figure 9.1
A custom control in
design mode
Start by renaming the UserControl1 object to FocusedTextBox. Renaming the object isn’t enough;
you must also rename the class that implements the control. Open the object’s code window (click
the View Code button at the top of the Solution Explorer while the UserControl1 object is selected)
and change the line
to
Public Class UserControl1
Public Class FocusedTextBox