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ByVal

Visual Studio 2008

Updated: August 2008

Specifies that an argument is passed in such a way that the called procedure or property cannot change the value of a variable underlying the argument in the calling code.

Remarks

The ByVal modifier can be used in these contexts:

Declare Statement

Function Statement (Visual Basic)

Operator Statement

Property Statement

Sub Statement (Visual Basic)

Example

The following example demonstrates the use of the ByVal parameter passing mechanism with a reference type argument. In the example, the argument is c1, an instance of class Class1. ByVal prevents the code in the procedures from changing the underlying value of the reference argument, c1, but does not protect the accessible fields and properties of c1.

Copy

Module Module1

    Sub Main()

        ' Declare an instance of the class and assign a value to its field.
        Dim c1 As Class1 = New Class1()
        c1.Field = 5
        Console.WriteLine("Original value for the field: " & c1.Field)

        ' ByVal does not prevent changing the value of a field or property.
        ChangeFieldValue(c1)
        Console.WriteLine("Value of field after ChangeFieldValue: " & c1.Field)

        ' ByVal does prevent changing the value of c1 itself. 
        ChangeClassReference(c1)
        Console.WriteLine("Value of field after ChangeClassReference: " & c1.Field)

    End Sub

    Public Sub ChangeFieldValue(ByVal cls As Class1)
        cls.Field = 500

    End Sub

    Public Sub ChangeClassReference(ByVal cls As Class1)
        cls = New Class1()
        cls.Field = 1000
    End Sub

    Public Class Class1
        Public Field As Integer
    End Class

End Module