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