How to: Protect a Procedure Argument Against Value Changes

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11/18/2006

If a procedure declares a parameter as ByRef, Visual Basic gives the procedure code a direct reference to the programming element underlying the argument in the calling code. This permits the procedure to change the value underlying the argument in the calling code. In some cases the calling code might want to protect against such a change.

You can always protect an argument from change by declaring the corresponding parameter ByVal in the procedure. If you want to be able to change a given argument in some cases but not others, you can declare it ByRef and let the calling code determine the passing mechanism in each call. It does this by enclosing the corresponding argument in parentheses to pass it by value, or not enclosing it in parentheses to pass it by reference. For more information, see How to: Force an Argument to Be Passed by Value.

Example

The following example shows two procedures that take an array variable and operate on its elements. The increase procedure simply adds one to each element. The replace procedure assigns a new array to the parameter a() and then adds one to each element. However, the reassignment does not affect the underlying array variable in the calling code.

Public Sub increase(ByVal a() As Long)
    For j As Integer = 0 To UBound(a)
        a(j) = a(j) + 1
    Next j
End Sub
Public Sub replace(ByVal a() As Long)
    Dim k() As Long = {100, 200, 300}
    a = k
    For j As Integer = 0 To UBound(a)
        a(j) = a(j) + 1
    Next j
End Sub
Dim n() As Long = {10, 20, 30, 40}
Call increase(n)
MsgBox("After increase(n): " & CStr(n(0)) & ", " & _
    CStr(n(1)) & ", " & CStr(n(2)) & ", " & CStr(n(3)))
Call replace(n)
MsgBox("After replace(n): " & CStr(n(0)) & ", " & _
    CStr(n(1)) & ", " & CStr(n(2)) & ", " & CStr(n(3)))

The first MsgBox call displays "After increase(n): 11, 21, 31, 41". Because the arraynis a reference type,replacecan change its members, even though the passing mechanism is ByVal.

The second MsgBox call displays "After replace(n): 11, 21, 31, 41". Becausenis passed ByVal,replacecannot modify the variablenin the calling code by assigning a new array to it. Whenreplacecreates the new array instancekand assigns it to the local variable a, it loses the reference tonpassed in by the calling code. When it changes the members of a, only the local arraykis affected. Therefore,replacedoes not increment the values of arraynin the calling code.

Compiling the Code

The default in Visual Basic is to pass arguments by value. However, it is good programming practice to include either the ByVal or ByRef keyword with every declared parameter. This makes your code easier to read.

共用方式為

How to: Protect a Procedure Argument Against Value Changes

Example

Compiling the Code

See Also

Tasks

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