Declaration and assignment of variables and constants in Visual Basic

Declaring variables

Variables can be declared either explicitly or implicitly in Visual Basic. To declare variable explicitly it is required to use Dim keyword or Public keyword to declare the variable as public class or module member (refer the Variables Scope article for more information).

Type of the variable can be declared using As keyword.

Dim textVal As String

If type of the variable is not explicitly stated than it is defaulted to Variant

Dim varVal 'implicitly declared as Variant

Dim keyword is not used when variable is declared implicitly. In this case value of the variable assigned directly.

implicitVal = 10 'Implicit declaration and assignment of the variable

This is not recommended approach as it may introduce the ambiguity and potential issues with the code.

In order to force explicit variable declaration in Visual Basic it is required to use Option Explicit statement. In this case compile error will occur if implicit variable is identified.

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Declaring group of variables of the same type

Variables can be declared in the group.

Dim textVar1, textVar2, textVar3 As String '3 variables explicitly declared as String

This approach allows to make code more readable and compact.

Declaring group of variables with different types

It is allowed to use declaration characters for each variable to declare the type explicitly using the short declaration

Dim intVar%, doubleVar#, longVar& 'Integer, Double and Long variables are declared explicitly using short declaration

Refer the Standard Types article for the list of declaration characters.

This is a legacy way to declare the variables. This approach is not recommended way to declare the variables.

Assigning the values

In order to assign the value of the variable it is required to use equal (=) sign, where the variable name appears on the left and variable value appears on the right.

Dim varName As String
varName = "VarValue"

it is possible to copy the value from one variable to another

Dim var1 As Integer
Dim var2 As Integer
var1 = 10
var2 = var1 'value of var2 now equals to var1 which equals to 10

It is possible to assign the value to the variable as the result of calling another function. Refer Functions and Procedures article on more information about functions.

Dim funcRes As Double
funcRes = GetValueFunc()

Declaring constants

Constant allows to define the value which will not change during execution. It is usually used for declaring mathematical constants (e.g. PI, gravitational constant, etc.), conversions factors (e.g. hours to minutes, inches to meters etc.) or any other program specific values.

Constant is declared using Const statement and must assign the value on declaration.

Const G As Double = 9.8 'gravitational constant

Same as variable declaration constant type can be defined explicitly (using As keyword) or implicitly.

Once declared value of the constant cannot be changed. Otherwise the compile error will occur.

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This code example demonstrates different ways of declaring and assigning constants and value variables.

Sub main()

    Dim i, j, k As Integer 'declaring 3 variables of type Integer

    i = 10 'setting the value to declared variable
    l = 20 'setting the value of implicitly declared variable
    Debug.Print TypeName(l) 'Integer
    
    Dim intVar%, doubleVar#, longVar&
    Debug.Print TypeName(intVar%) 'Integer
    Debug.Print TypeName(doubleVar#) 'Double
    Debug.Print TypeName(longVar&) 'Long
    
    Const PI As Double = 3.14159265359 'declaring and initiating constant
    'PI = 0 'compile error
    
    Dim res As Double
    res = Sqrt(16) 'returns 4 and assigns to res variable
    Debug.Print res
    
End Sub

Assigning reference variables

Unlike value types, references types must follow several additional rules when assigning the value.

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• It is required to use new keyword to create new instance of the referenced type. Otherwise Run-time error '91' will be displayed

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• It is required to use Set keyword to assign the value, otherwise the Run-time error '91' will be displayed

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See code below for the correct assignment of reference type variable.

Sub main()

    Dim userType As MyUserType
    Set userType = New MyUserType
    Dim obj As Object
    
    'obj = userType 'Object variable or with block variable not set when Set keyword is not used
    Set obj = userType 'assigning the pointer to the MyUserType object to obj variable

End Sub

References variables are only holding the pointer to the actual value, i.e. Set keyword assigns the reference (not the actual value like in value types). That means if reference of one variable is assigned to another variables, both of them now refer the same data.

Binding

Binding is a process of assigning object to a variable. When early binding is used the specific object type is declared in advanced so the binding can occur at compile time. Late binding is resolved at runtime and specific object type is not known in advance.

Early Binding

Dim objLate As Object 'example of late binding
Dim objEarly as MySpecificType 'example of early binding

Early bound objects are usually initiated with new keyword

Dim objEarly as MySpecificType
Set objEarly = new MySpecificType

Benefits

• Performance. Compiler can perform required optimization as the type of the object and its size is known at compile time
• Maintainability. Code is cleaner and easier to maintain and read when specific type is declared
• Dynamic help and IntelliSense (code completion) features are available for early bound objects

Late Binding

While late bound objects are usually initiated with CreateObject or GetObject functions

Dim xlApp As Object
Set xlApp = CreateObject("Excel.Application")

But it is still acceptable to use new keyword in late binding and CreateObject or GetObject in early binding.

Benefits

• No need to maintain 3rd party references which may be an issue when code is ported to another environment or another version of 3rd party references is released. Refer this Example of references issue