Variables, Data Types, and Constants (Visual Basic)

Overview

Visual Basic provides a rich set of keywords for declaring variables, specifying data types, and defining constants. Understanding these concepts is essential for building robust .NET applications.

Declaring Variables

Use the Dim statement to declare a variable. The As clause specifies its data type.

Dim counter As Integer
Dim message As String
Dim isValid As Boolean = True
Dim data() As Byte = New Byte(9) {0,1,2,3,4,5,6,7,8,9}

You can also declare multiple variables of the same type in one line:

Dim x, y, z As Double

Data Types

VB.NET includes both built‑in and .NET Framework types. The table below lists the most common types.

VB Type	.NET Type	Size	Range / Description
Boolean	System.Boolean	1 byte	True or False
Byte	System.Byte	1 byte	0 to 255
Char	System.Char	2 bytes	Unicode character
Decimal	System.Decimal	16 bytes	28‑28 significant digits
Double	System.Double	8 bytes	±5.0 × 10⁻³² to ±1.7 × 10³⁸
Integer	System.Int32	4 bytes	‑2,147,483,648 to 2,147,483,647
Long	System.Int64	8 bytes	‑9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
Object	System.Object	Reference	Base type of all .NET types
String	System.String	Reference	Unicode text
Single	System.Single	4 bytes	±1.5 × 10⁻⁴⁵ to ±3.4 × 10³⁸
UInteger	System.UInt32	4 bytes	0 to 4,294,967,295
UShort	System.UInt16	2 bytes	0 to 65,535
Structure	System.ValueType	Varies	User‑defined value type

Option Strict

Enable Option Strict On to enforce early binding and disallow implicit narrowing conversions. This helps catch type‑related errors at compile time.

Option Strict On

Dim i As Integer = 10
Dim d As Double = i   ' OK – widening conversion
'Dim s As String = i   ' Error: Option Strict On disallows implicit conversion

Constants

Use the Const keyword for values that never change.

Const Pi As Double = 3.14159265358979
Const MaxItems As Integer = 100

Constants are implicitly Shared and must be initialized at declaration.

Complete Example

The following example demonstrates variable declaration, type conversion, and constant usage.

Option Strict On
Module Module1
    Const Greeting As String = "Hello, "
    Sub Main()
        Dim name As String = "World"
        Dim times As Integer = 3
        Dim result As String = Greeting & name

        For i As Integer = 1 To times
            Console.WriteLine(result)
        Next

        ' Demonstrate conversion
        Dim age As Byte = 30
        Dim ageDouble As Double = CDbl(age)   ' widening conversion
        Console.WriteLine($"Age as Double: {ageDouble}")

        Console.ReadLine()
    End Sub
End Module