C# 4 and .NET 4

summary ❘ 1147
Creating your own user Controls
In addition to using the built-in controls, it is also possible to create your own custom controls and then
reuse these in your own applications. There are two types of custom controls — those derived from the
UserControl base class and those derived from Control. A UserControl is the simplest type to create,
as you can compose one from the other controls in the toolbox. Typically, you’ll write a custom control
only when the built-in controls cannot be used to do what you want. Entire books have been written on the
creation of custom controls, so here we’ll concentrate on user controls.
In this section, you’ll build the user control shown in
Figure 39-10, which includes an error provider validator,
and exposes its fields for reuse. The code for this
example is available in the 02 UserControl folder of
the chapter downloads.
To begin you need to construct a new user control,
which can be done from the Solution Explorer or the
project menu. Once it is created, you’ll have a class that
derives from UserControl and can then drag and drop
other controls onto the design surface. In this example
you use a set of label controls and a set of text boxes to
replicate the functionality shown in Figure 39-10.
Once the UI has been laid out, you want to define anchor behavior for the controls. This is important to do
as it permits the user control to resize properly when its size is changed. In the example control, each of the
text boxes has had its alignment set to Left + Top + Right, and a small space on the right of the list boxes
has been provided, which will show any validation errors.
In addition to the text boxes, an ErrorProvider object has been added to the control so that validation
errors can be shown while the user tabs around the control. The first two address fields and the postcode
have validation errors assigned, so they are mandatory.
Once you are happy with the design and implementation of your control, you can simply compile it, and it
will be displayed within the tool palette in Visual Studio. You can then reuse that control in any application
that needs it.
summary
figure 39-10
This chapter has given you the basics for building Windows-based client applications. It explained many of
the basic controls by discussing the hierarchy of the Windows.Forms namespace and examining the various
properties and methods of the controls.
The chapter also showed you how to create a basic user control. The power and flexibility of creating
your own controls cannot be emphasized enough. By creating your own toolbox of custom controls,
Windows-based client applications will become easier to develop and to test because you will be reusing
the same tested components over and over again.
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