In this lesson, we shall learn some basic techniques in writing the Visual Basic program code. First of all, we should understand that each control or object in VB are able to run numerous kinds of events. These events are listed in the dropdown list in the code window, that is displayed when you double-click on an object and click on the procedures’ box. Among the events are loading a form, clicking on a command button, pressing a key on the keyboard or dragging an object and more. For each event, you need to write an event procedure so that it can perform an action or a series of actions.
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To start writing code for an event procedure, you need to double-click an object to enter the VB code window. For example, if you want to write code for the event of clicking a command button, you double-click the command button and enter the codes in the event procedure that appears in the code window, as shown in Figure 4.1.
Visual Basic is a better programming language than Visual C#. I can do this in Visual Basic without leaving the code window. Here are a couple of functions I use from time to time in VB. If you want to dynamically change the length of an array in Visual Basic, you can use Redim Preserve.
The structure of an event procedureis as follows:
You enter the codes in the space between Private Sub Command1_Click............. End Sub.The keyword Sub actually stands for a sub procedure that made up a part of all the procedures in a program or a module. The program code is made up of a number of VB statements that set certain properties or trigger some actions. The syntax of the Visual Basic’s program code is almost like the normal English language, though not exactly the same, so it is fairly easy to learn.
The syntax to set the property of an object or to pass a certain value to it is :
where Object and Property are separated by a period (or dot). For example, the statement Form1.Show means to show the form with the name Form1, Iabel1.Visible=true means label1 is set to be visible, Text1.text=”VB” is to assign the text VB to the text box with the name Text1, Text2.text=100 is to pass a value of 100 to the text box with the name text2, Timer1.Enabled=False is to disable the timer with the name Timer1 and so on. Let’s examine a few examples below:
Example 4.1Example 4.2Example 4.3Example 4.4 A Counter
This is a counter which start counting after the user click on a command button. In this program, we insert a label, two command buttons and a Timer control. The label acts as a counter, one of the command buttons is to start the counter and the other one is to stop the counter. The Timer control is a control that is only used by the developer, it is invisible during runtime and it does not allow the user to interact with it.
The Timer's Interval property determine how frequent the timer changes. A value of 1 is 1 milliseconds which means a value of 1000 represents 1 second. In this example, we set the interval to 100, which represents 0.1 second interval. In addition, the Timer's Enabled property is set to false at design time as we do not want the program to start counting immediately, the program only start counting after the the user clicks on te 'Start Counting' button. You can also reset the counter using another command button.
The Code
* We declare the variable n in the general area. After the Timer1 is enabled, it will add 1 to the preceding number using n=n+1 after every interval untill the user click on the 'Stop Counting' button.
The OutputExample 4.5 Click and Double Click
This program display a message whether the label is being click once or click twice. In this program, insert a label and rename it as MyLabel and change its caption to 'CLICK ME'. Next, key in the following codes:
The Output
Running the program and click the label once, the 'CLICK ME' caption will change to 'You Click Me Once'. If you click the label twice, the 'CLICK ME' caption will change to 'You Click Me Twice!'.
In Visual Basic, most of the syntaxes resemble the English language. Among the syntaxes are Print, If…Then….Else….End If, For…Next, Select Case…..End Select , End and Exit Sub. For example, Print “ Visual Basic” is to display the text Visual Basic on screen and End is to end the program.
Program code that involves calculations is fairly easy to write, just like what you do in mathematics. However, in order to write an event procedure that involves calculations, you need to know the basic arithmetic operators in VB as they are not exactly the same as the normal operators , except for + and - .
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For multiplication, we use *, for division we use /, for raising a number x to the power of n, we use x ^n and for square root, we use Sqr(x). VB offers many more advanced mathematical functions such as Sin, Cos, Tan and Log, they will be discussed in lesson 10. There are also two important functions that are related to arithmetic operations, i.e. the functions Val and Str$ where Val is to convert text to a numerical value and Str$ is to convert numerical to a string (text). While the function Str$ is as important as VB can display a numeric value as string implicitly, failure to use Val will result in the wrong calculation. Let’s examine Example 4.4 and example 4.5.
Example 4.4Example 4.5
When you run the program in example 4.4 and enter 12 in textbox1 and 3 in textbox2 will give you a result of 123, which is wrong. It is because VB treat the numbers as string and so it just joins up the two strings. On the other hand, running exampled 4.5 will give you the correct result, i.e., 15.
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VB.Net is an object-oriented programming language. In Object-Oriented Programming methodology, a program consists of various objects that interact with each other by means of actions. The actions that an object may take are called methods. Objects of the same kind are said to have the same type or, more often, are said to be in the same class.
When we consider a VB.Net program, it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what do class, object, methods and instance variables mean.
A Rectangle Class in VB.Net
For example, let us consider a Rectangle object. It has attributes like length and width. Depending upon the design, it may need ways for accepting the values of these attributes, calculating area and displaying details.
Let us look at an implementation of a Rectangle class and discuss VB.Net basic syntax on the basis of our observations in it −
When the above code is compiled and executed, it produces the following result −
In previous chapter, we created a Visual Basic module that held the code. Sub Main indicates the entry point of VB.Net program. Here, we are using Class that contains both code and data. You use classes to create objects. For example, in the code, r is a Rectangle object.
An object is an instance of a class −
A class may have members that can be accessible from outside class, if so specified. Data members are called fields and procedure members are called methods.
Shared methods or static methods can be invoked without creating an object of the class. Instance methods are invoked through an object of the class −
Identifiers
An identifier is a name used to identify a class, variable, function, or any other user-defined item. The basic rules for naming classes in VB.Net are as follows −
VB.Net Keywords
The following table lists the VB.Net reserved keywords −
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