Scope in Visual Basic

Artykuł
02/04/2013

The scope of a declared element is the set of all code that can refer to it without qualifying its name or making it available through an Imports Statement (.NET Namespace and Type). An element can have scope at one of the following levels:

Level	Description
Block scope	Available only within the code block in which it is declared
Procedure scope	Available to all code within the procedure in which it is declared
Module scope	Available to all code within the module, class, or structure in which it is declared
Namespace scope	Available to all code in the namespace in which it is declared

These levels of scope progress from the narrowest (block) to the widest (namespace), where narrowest scope means the smallest set of code that can refer to the element without qualification. For more information, see "Levels of Scope" on this page.

Specifying Scope and Defining Variables

You specify the scope of an element when you declare it. The scope can depend on the following factors:

The region (block, procedure, module, class, or structure) in which you declare the element
The namespace containing the element's declaration
The access level you declare for the element

Use care when you define variables with the same name but different scope, because doing so can lead to unexpected results. For more information, see References to Declared Elements (Visual Basic).

Levels of Scope

A programming element is available throughout the region in which you declare it. All code in the same region can refer to the element without qualifying its name.

Block Scope

A block is a set of statements enclosed within initiating and terminating declaration statements, such as the following:

Do and Loop
For [Each] and Next
If and End If
Select and End Select
SyncLock and End SyncLock
Try and End Try
While and End While
With and End With

If you declare a variable within a block, you can use it only within that block. In the following example, the scope of the integer variable cube is the block between If and End If, and you can no longer refer to cube when execution passes out of the block.

If n < 1291 Then
    Dim cube As Integer
    cube = n ^ 3
End If

Note

Even if the scope of a variable is limited to a block, its lifetime is still that of the entire procedure. If you enter the block more than once during the procedure, each block variable retains its previous value. To avoid unexpected results in such a case, it is wise to initialize block variables at the beginning of the block.

Procedure Scope

An element declared within a procedure is not available outside that procedure. Only the procedure that contains the declaration can use it. Variables at this level are also known as local variables. You declare them with the Dim Statement (Visual Basic), with or without the Static (Visual Basic) keyword.

Procedure and block scope are closely related. If you declare a variable inside a procedure but outside any block within that procedure, you can think of the variable as having block scope, where the block is the entire procedure.

Note

All local elements, even if they are Static variables, are private to the procedure in which they appear. You cannot declare any element using the Public (Visual Basic) keyword within a procedure.

Module Scope

For convenience, the single term module level applies equally to modules, classes, and structures. You can declare elements at this level by placing the declaration statement outside of any procedure or block but within the module, class, or structure.

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

Elements for which you declare Private (Visual Basic) access level are available to every procedure in that module, but not to any code in a different module. The Dim statement at module level defaults to Private if you do not use any access level keywords. However, you can make the scope and access level more obvious by using the Private keyword in the Dim statement.

In the following example, all procedures defined in the module can refer to the string variable strMsg. When the second procedure is called, it displays the contents of the string variable strMsg in a dialog box.

' Put the following declaration at module level (not in any procedure).
Private strMsg As String
' Put the following Sub procedure in the same module.
Sub initializePrivateVariable()
    strMsg = "This variable cannot be used outside this module."
End Sub
' Put the following Sub procedure in the same module.
Sub usePrivateVariable()
    MsgBox(strMsg)
End Sub

Namespace Scope

If you declare an element at module level using the Friend (Visual Basic) or Public (Visual Basic) keyword, it becomes available to all procedures throughout the namespace in which the element is declared. With the following alteration to the preceding example, the string variable strMsg can be referred to by code anywhere in the namespace of its declaration.

' Include this declaration at module level (not inside any procedure).
Public strMsg As String

Namespace scope includes nested namespaces. An element available from within a namespace is also available from within any namespace nested inside that namespace.

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 project scope. Public elements in a module, class, or structure are also available to any project that references their project.

Choice of Scope

When you declare a variable, you should keep in mind the following points when choosing its scope.

Advantages of Local Variables

Local variables are a good choice for any kind of temporary calculation, for the following reasons:

Name Conflict Avoidance. Local variable names are not susceptible to conflict. For example, you can create several different procedures containing a variable called intTemp. As long as each intTemp is declared as a local variable, each procedure recognizes only its own version of intTemp. Any one procedure can alter the value in its local intTemp without affecting intTemp variables in other procedures.
Memory Consumption. Local variables consume memory only while their procedure is running. Their memory is released when the procedure returns to the calling code. By contrast, Shared (Visual Basic) and Static (Visual Basic) variables consume memory resources until your application stops running, so use them only when necessary. Instance variables consume memory while their instance continues to exist, which makes them less efficient than local variables, but potentially more efficient than Shared or Static variables.

Minimizing Scope

In general, when declaring any variable or constant, it is good programming practice to make the scope as narrow as possible (block scope is the narrowest). This helps conserve memory and minimizes the chances of your code erroneously referring to the wrong variable. Similarly, you should declare a variable to be Static (Visual Basic) only when it is necessary to preserve its value between procedure calls.

Udostępnij za pośrednictwem