Variables and Constants

Variables are specific areas allocated in computer memory for storing data. A simple way to think about variables is to imagine mailboxes in a post office. The contents of a mailbox can change over time, and the same is true for the value stored in a variable.

In Visual Basic 2015, every variable must have a name. This name allows you to refer to the stored value later in your code.

To create a valid variable name in Visual Basic 2015, you need to follow a few rules:

• It must be less than 255 characters.
• No spaces are allowed.
• It must not begin with a number.
• A period is not allowed.

Good variable names make your code easier to read and understand.

In Visual Basic 2015, you usually declare variables before using them. To declare a variable, you provide its name and its data type.

Dim VariableName As DataType

If you want to declare more than one variable, you can declare them on separate lines or combine them into one line.

Dim VariableName1 As DataType1, VariableName2 As DataType2, VariableName3 As DataType3

Declaring variables clearly tells Visual Basic what kind of values those variables are expected to store.

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim password As String
    Dim MyName As String
    Dim Num1 As Integer
    Dim Num2 As Single
    Dim Sum As Integer
    Dim StartDate As Date
End Sub

You can also write them in one line:

Dim password As String, MyName As String, Num1 As Integer, Num2 As Single, Sum As Integer, StartDate As Date

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim YourName As String = "George"
    Dim MyMsg As String
    MyMsg = "Happy Birthday!"
    MsgBox(MyMsg & "," & YourName)
End Sub

Notice that the following two approaches mean the same thing:

Dim YourName As String = "George"

and

Dim YourName As String
YourName = "George"

When the program runs, the message box will show the text Happy Birthday!, George.

After declaring variables, you can assign values to them. The basic syntax is:

Variable = Expression

The expression can be a number, a string, a mathematical formula, a Boolean value, or the value of another variable or control property.

firstNumber = 100
secondNumber = firstNumber - 99
userName = "John Lyan"
userpass.Text = password
Label1.Visible = True
Command1.Visible = False
Label4.Text = TextBox1.Text
ThirdNumber = Val(usernum1.Text)
total = firstNumber + secondNumber + ThirdNumber
X = Sqr(16)
TrimString = LTrim(" Visual Basic")
Num = Int(Rnd * 6) + 1

An error occurs if you assign a value that does not match the declared data type of the variable. For example, if you declare a variable as an integer but then assign a string to it, Visual Basic will report an error.

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim YourMessage As Integer
    YourMessage = "Happy Birthday!"
    MsgBox(YourMessage)
End Sub

This happens because an integer variable cannot store a sentence.

Although Dim is the most common keyword for declaration, Visual Basic 2015 also allows other keywords such as Private, Static, and Public.

Private VariableName As DataType
Static VariableName As DataType
Public VariableName As DataType

• Private declares a variable local to a procedure or module.
• Static preserves the variable’s value even after the procedure ends.
• Public declares a global variable that can be used across the program.

Scope tells you where a variable can be used and how long its value will remain available.

Constants are similar to variables, but their values do not change while the program is running. They are useful for fixed values such as mathematical constants, tax rates, and conversion factors.

Const ConstantName As DataType = Value

Here is an example using a constant for the value of Pi:

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Const Pi As Single = 3.142
    Dim R As Single = 10
    Dim AreaCircle As Single
    AreaCircle = Pi * R ^ 2
    MsgBox("Area of circle with " & "radius" & R & "=" & AreaCircle)
End Sub

In this example, Pi is declared as a constant because its value should not change.