How to: Control the Scope of a Variable (Visual Basic)

Articolo
11/24/2013

Normally, a variable is in scope, or visible for reference, throughout the region in which you declare it. In some cases, the variable's access level can influence its scope.

For more information, see Scope in Visual Basic.

Scope at Block or Procedure Level

To make a variable visible only within a block

Place the Dim Statement (Visual Basic) for the variable between the initiating and terminating declaration statements of that block, for example between the For and Next statements of a For loop.

You can refer to the variable only from within the block.

To make a variable visible only within a procedure

Place the Dim statement for the variable inside the procedure but outside any block (such as a With...End With block).

You can refer to the variable only from within the procedure, including inside any block contained in the procedure.

Scope at Module or Namespace Level

For convenience, the single term module level applies equally to modules, classes, and structures. The access level of a module level variable determines its scope. The namespace that contains the module, class, or structure also influences the scope.

To make a variable visible throughout a module, class, or structure

Place the Dim statement for the variable inside the module, class, or structure, but outside any procedure.
Include the Private (Visual Basic) keyword in the Dim statement.
You can refer to the variable from anywhere within the module, class, or structure, but not from outside it.

To make a variable visible throughout a namespace

Place the Dim statement for the variable inside the module, class, or structure, but outside any procedure.
Include the Friend (Visual Basic) or Public (Visual Basic) keyword in the Dim statement.
You can refer to the variable from anywhere within the namespace containing the module, class, or structure.

Example

The following example declares a variable at module level and limits its visibility to code within the module.

Module demonstrateScope
    Private strMsg As String
    Sub initializePrivateVariable()
        strMsg = "This variable cannot be used outside this module."
    End Sub
    Sub usePrivateVariable()
        MsgBox(strMsg)
    End Sub
End Module

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

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.

    Public strMsg As String

Robust Programming

The narrower the scope of a variable, the fewer opportunities you have for accidentally referring to it in place of another variable with the same name. You can also minimize problems of reference matching.

Security

The narrower the scope of a variable, the smaller the chances that malicious code can make improper use of it.

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