- Control Structures
- If/Then Selection Structure
- If/Then/Else Selection Structure
- While Repetition Structure
- DoWhile/Loop Repetition Structure
- DoUntil/Loop Repetition Structure
- Do/LoopWhile Repetition Structure
- Do/LoopUntil Repetition Structure
- Assignment Operators
- For/Next Repetition Structure
- Example: Using the For/Next Structure to Compute Compound Interest
- SelectCase Multiple-Selection Structure
- Using the Exit Keyword in a Repetition Structure
- Logical Operators
- Introduction to Windows Application Programming
3.16 Introduction to Windows Application Programming
Today, users demand software with rich graphical user interfaces (GUIs) that allow them to click buttons, select items from menus and much more. In this chapter and the previous one, we have created console applications. However, the vast majority of Visual Basic programs used in industry are Windows applications with GUIs. For this reason, we have chosen to introduce Windows applications early in the book, although doing so exposes some concepts that cannot be explained fully until later chapters.
In Chapter 2, we introduced the concept of visual programming, which allows programmers to create GUIs without writing any program code. In this section, we combine visual programming with the conventional programming techniques introduced in this chapter and the previous chapter. Through this combination, we can enhance considerably the Windows application introduced in Chapter 2.
Before proceeding, load the project ASimpleProgram from Chapter 2 into the IDE, and change the (Name) properties of the form, label and picture box to FrmASim-pleProgram, lblWelcome and picBug, respectively. The modification of these names enables us easily to identify the form and its controls in the program code. [Note: In this section, we change the file name from Form1.vb to ASimpleProgram.vb, to enhance clarity.]
Good Programming Practice 3.2
The prefixes Frm, lbl and pic allow forms, labels and picture boxes; respectively, easily to be identified easily in program code.
With visual programming, the IDE generates the program code that creates the GUI. This code contains instructions for creating the form and every control on it. Unlike with a console application, a Windows application's program code is not displayed initially in the editor window. Once the program's project (e.g., ASimpleProgram) is opened in the IDE, the program code can be viewed by selecting View > Code. Figure 3.16 shows the code editor displaying the program code.
Fig. 3.16 DE showing code for the program inI Fig. 2.14.
Notice that no module is present. Instead, Windows applications use classes. We already have seen examples of classes, such as Console and MessageBox, which are defined within the .NET Framework Class Library (FCL). Like modules, classes are logical groupings of procedures and data that simplify program organization. Modules are discussed in detail in Chapter 4, Procedures. In-depth coverage of classes is provided in Chapter 5, Object-Based Programming.
Every Windows application consists of at least one class that Inherits from class Form (which represents a form) in the FCL's System.Windows.Forms namespace. The keyword Class begins a class definition and is followed immediately by the class name (FrmASimpleProgram). Recall that the form's name is set by means of the (Name) property. Keyword Inherits indicates that the class FrmASimplePro-gram inherits existing pieces from another class. The class from which FrmASimpleProgram inheritshere, System.Win-dows.Forms.Formappears to the right of the Inherits keyword. In this inheritance relationship, Form is called the superclass, or base class, and FrmASimpleProgram is called the subclass, or derived class. The use of inheritance results in a FrmASimpleProgram class definition that has the attributes (data) and behaviors (methods) of class Form. We discuss the significance of the keyword Public in Chapter 5.
A key benefit of inheriting from class Form is that someone else previously has defined "what it means to be a form." The Windows operating system expects every window (e.g., a form) to have certain capabilities (attributes and behaviors). However, because class Form already provides those capabilities, programmers do not need to "reinvent the wheel" by defining all those capabilities themselves. In fact, class Form has over 400 methods! In our programs up to this point, we have used only one method (i.e., Main), so you can imagine how much work went into creating class Form. The use of Inherits to extend from class Form enables programmers to create forms quickly and easily.
In the editor window (Fig. 3.16), notice the text WindowsFormDesignergen-eratedcode, which is colored gray and has a plus box next to it. The plus box indicates that this section of code is collapsed. Although collapsed code is not visible, it is still part of the program. Code collapsing allows programmers to hide code in the editor, so that they can focus on key code segments. Notice that the entire class definition also can be collapsed, by clicking the minus box to the left of Public. In Fig 3.16, the description to the right of the plus box indicates that the collapsed code was created by the Windows Form Designer (i.e., the part of the IDE that creates the code for the GUI). This collapsed code contains the code created by the IDE for the form and its controls, as well as code that enables the program to run. Click the plus box to view the code.
Upon initial inspection, the expanded code (Fig. 3.17) appears complex.This code is created by the IDE and normally is not edited by the programmer. However, we feel that it is important for readers to see the code that is generated by the IDE, even though much of the code is not explained until later in the book. This type of code is present in every Windows application. Allowing the IDE to create this code saves the programmer considerable development time. If the IDE did not provide the code, the programmer would have to write it, which would require a considerable amount of time. The vast majority of the code shown has not been introduced yet, so you are not expected to understand how it works. However, certain programming constructs, such as comments and control structures, should be familiar. Our explanation of this code will enable us to discuss visual programming in greater detail. As you continue to study Visual Basic, especially in Chapters 510, the purpose of this code will become clearer.
Fig. 3.17 Windows Form Designer-generated code when expanded.
When we created this application in Chapter 2, we used the Properties window to set properties for the form, label and picture box. Once a property was set, the form or control was updated immediately. Forms and controls contain a set of default properties, which are displayed initially in the Properties window when a form or control is created. These default properties provide the initial characteristics of a form or control when it is created. When a control, such as a label, is placed on the form, the IDE adds code to the class (e.g., FrmASimpleProgram) that creates the control and that sets some of the control's property values, such as the name of the control and its location on the form. Figure 3.18 shows a portion of the code generated by the IDE for setting the label's (i.e., lblWelcome's) properties, including the label's Font, Location, Name, Text and TextAlign properties. Recall from Chapter 2 that we explicitly set values for the label's Text and Tex-tAlign properties. Other properties, such as Location, are set only when the label is placed on the form.
Fig. 3.18 Code generated by the IDE for lblWelcome.
The values assigned to the properties are based on the values in the Properties window. We now demonstrate how the IDE updates the Windows Form Designergenerated code created when a property value in the Properties window changes. During this process, we must switch between code view and design view. To switch views, select the corresponding tabs: ASimpleProgram.vb for code view and ASimpleProgram.vb [Design] for design view. Alternatively, you can select View > Code or View > Designer. Perform the following steps:
Modify the file name. First, change the name of the file from Form1.vb to ASimpleProgram.vb by clicking the file name in the Solution Explorer and changing the File Nameproperty.
Modify the label control's Textproperty, using the Properties window. Recall that properties can be changed in design view by clicking a form or control to select it and modifying the appropriate property in the Properties window. Change the Text property of the label to "DeitelandAssociates" (Fig. 3.19).
Fig. 3.19 Properties window as used to set a property value.
Examine the changes in code view. Switch to code view, and examine the code. Notice that the label's Textproperty is now assigned the text that we entered in the Properties window (Fig. 3.20). When a property is changed in design mode, the Windows Form Designer updates the appropriate line of code in the class to reflect the new value.
Fig. 3.20 Windows Form Designer-generated code reflecting new property values.
Modify a property value in code view. In the code-view editor, locate the three lines of comments indicating the initialization for lblWelcome, and change the Stringassigned to Me.lblWelcome.Textfrom "Deitel andAssoci-ates" to "VisualBasic.NET" (Fig. 3.21).Then switch to design mode. The label now displays the updated text, and the Properties window for lblWel-comedisplays the new Textvalue (Fig. 3.22).
Property values should not be set using the techniques presented in this step. Here, we modify the property value in the IDE-generated code only as a demonstration of the relationship between program code and the Windows Form Designer.
Fig. 3.21 Changing a property in the code-view editor.
Fig. 3.22 New Text property value as reflected in design mode.
Change the label's Textproperty at runtime. In the previous steps, we set properties at design time. Often, however, it is necessary to modify a property while a program is running. For example, to display the result of a calculation, a label's text can be assigned a Stringcontaining the result. In console applications, such code is located in Main. In Windows applications, we must create a method that executes when the form is loaded into memory during program execution. Like Main, this method is invoked when the program is run. Double-clicking the form in design view adds a method named FrmASimpleProgram_Load to the class (Fig. 3.23). Notice that FrmASimpleProgram_Loadis not part of the Windows Form Designer-generated code. Add the statement lblWelcome.Text="Vi-sualBasic"into the body of the method definition (Fig. 3.24). In Visual Basic, properties are accessed by placing the property name (i.e., Textin this case) after the class name (i.e., lblWelcomein this case), separated by the dot operator. This syntax is similar to that used when accessing class methods. Notice that the Intel-liSense feature displays the Textproperty in the member list after the class name and dot operator have been typed (Fig. 3.23). In Chapter 5, Object-Based Programming, we discuss how programmers can create their own properties.
Fig. 3.23 Adding program code to FrmASimpleProgram_Load.
Fig. 3.24 Method FrmASimpleProgram_Load containing program code.
Examine the results of the FrmASimpleProgram_Load method. Notice that the text in the label looks the same in Design mode as it did in Fig. 3.22. Note also that the Property window still displays the value "VisualBasic.NET" as the label's Textproperty. The IDE-generated code has not changed either. Select Build > Build Solution and Debug > Start to run the program. Once the form is displayed, the text in the label reflects the property assignment in FrmASimpleProgram_Load(Fig. 3.25).
Fig. 3.25 Changing a property value at runtime.
Terminate program execution. Click the close button to terminate program execution. Once again, notice that both the label and the label's Textproperty contain the text Visual Basic .NET. The IDE-generated code also contains the text VisualBasic.NET, which is assigned to the label's Textproperty.
This chapter discussed how to compose programs from control structures that contain actions and decisions. In Chapter 4, Procedures and Arrays, we introduce another program-structuring unit, called the procedure. We will discuss how to compose programs by combining procedures that are composed of control structures. We also discuss how procedures promote software reusability. In Chapter 5, Object-Based Programming, we discuss in more detail another Visual Basic program-structuring unit, called the class. We then create objects from classes and proceed with our treatment of object-oriented programminga key focus of this book.