Understanding Scope in VB.net

By: Steven Holzner Viewed: 169 times  Printer Friendly Format    


The scope of a variable or constant is the set of all code that can refer to it without qualifying its name. A variable's scope is determined by where the variable is declared. It's usually a good idea to make the scope of variables or constants as narrow as possible (block scope is the narrowest). This helps conserve memory and minimizes the chances of your code referring to the wrong item. I'll take a look at the different kinds of scope in VB .NET here.

Block Scope

A block is a series of statements terminated by an End, Else, Loop, or Next statement, and an element declared within a block can be used only within that block. Here's what block scope looks like in an example. In this case, I'll declare a variable, strText in an If statement. That variable can be used inside the If statement's block, but not outside (VB .NET will tag the second use here as a syntax error):

Module Module1
    Sub Main()
        Dim intValue As Integer = 1
        If intValue = 1 Then
            Dim strText As String = "No worries."
            System.Console.WriteLine(strText)
        End If
        System.Console.WriteLine(strText)         'Will not work!
     End Sub
End Module

Procedure Scope

An element declared within a procedure is not available outside that procedure, and only the procedure that contains the declaration can use it. Elements at this level are also called local elements, and you declare them with the Dim or Static statement.

Note also that if an element is declared inside a procedure but outside any block within that procedure, the element can be thought of as having block scope, where the block is the entire procedure.

Module Scope

When discussing scope, Visual Basic uses the term module level to apply equally to modules, classes, and structures. You can declare elements at this level by placing the declaration statement outside of any procedure or block within the module, class, or structure.

When you make a declaration at the module level, the accessibility you choose determines the scope. The namespace that contains the module, class, or structure also affects the scope.

Elements for which you declare Private accessibility are available for reference to every procedure in that module, but not to any code in a different module. The Dim statement at module level defaults to Private accessibility, so it is equivalent to using the Private statement. However, you can make the scope and accessibility more obvious by using Private. In this example, I've declared Function1 as private to Module2, so it's inaccessible in Module1 (VB .NET will tag Module2.Function1 below as a syntax error):

Module Module1
    Sub Main()
        System.Console.WriteLine(Module2.Function1())   'Will not work!
    End Sub
End Module

Module Module2
    Private Function Function1() As String
        Return "Hello from Visual Basic"
    End Function

End Module

Namespace Scope

If you declare an element at module level using the Friend or Public statement, it becomes available to all procedures throughout the entire namespace in which it is declared. Note that an element accessible in a namespace is also accessible from inside any namespace nested inside that namespace.

Note 

If your project does not contain any namespace statements, everything in the project is in the same namespace. In this case, namespace scope can be thought of as procedure scope.



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