Visual Basic.NET How to Program (PDF)

Chapter 13 Graphical User Interfaces Concepts: Part 2 577
Visual inheritance enables developers to achieve visual consistency across applications
by reusing code. For example, a company could define a base form that contains a product’s
logo, a static background color, a predefined menu bar and other elements. Programmers then
could use the base form throughout an application for purposes of uniformity and branding.
Class FrmInheritance (Fig. 13.39) is a derived class of class Form. The output
depicts the workings of the program. The GUI contains two labels with text Bugs, Bugs,
Bugs and Copyright 2002, by Bug2Bug.com., as well as one button displaying the
text Learn More. When a user presses the Learn More button, method
cmdLearn_Click (lines 16–22) is invoked. This method displays a message box that
provides some informative text.
To allow other forms to inherit from FrmInheritance, we must package FrmInheritance
as a .dll. Right click the project's name in the Solution Explorer and
choose Properties. Under Common Properties > General, change Output Type to
Class Library. Building the project produces the .dll.
To visually inherit from FrmInheritance, we create an empty project. From the
Project menu, select Add Inherited Form... to display the Add New Item dialog.
Select Inherited Form from the Templates pane. Clicking OK displays the Inheritance
Picker tool. The Inheritance Picker tool enables programmers to create a form
which inherits from a specified form. Click button Browse and select the .dll file corresponding
to FrmInheritance. This .dll file normally is located within the project’s
bin directory. Click OK. The Form Designer should now display the inherited form
(Fig. 13.40). We can add components to the form.
1 ' Fig. 13.39: FrmInheritance.vb
2 ' Form template for use with visual inheritance.
3
4 Imports System.Windows.Forms
5
6 Public Class FrmInheritance
7 Inherits Form
8
9 Friend WithEvents lblBug As Label ' top label
10 Friend WithEvents lblCopyright As Label ' bottom label
11 Friend WithEvents cmdLearn As Button ' left button
12
13 ' Visual Studio .NET generated code
14
15 ' invoked when user clicks Learn More button
16 Private Sub cmdLearn_Click(ByVal sender As System.Object, _
17 ByVal e As System.EventArgs) Handles cmdLearn.Click
18
19 MessageBox.Show("Bugs, Bugs, Bugs is a product of " & _
20 " Bug2Bug.com.", "Learn More", MessageBoxButtons.OK, _
21 MessageBoxIcon.Information)
22 End Sub ' cmdLearn_Click
23
24 End Class ' FrmInheritance
Fig. 13.39 Class FrmInheritance, which inherits from class Form, contains a
button (Learn More) (part 1 of 2).