ByVal (Visual Basic)
Visual Studio 2012
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.
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.
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(c1.Field) ' Output: 5 ' ByVal does not prevent changing the value of a field or property. ChangeFieldValue(c1) Console.WriteLine(c1.Field) ' Output: 500 ' ByVal does prevent changing the value of c1 itself. ChangeClassReference(c1) Console.WriteLine(c1.Field) ' Output: 500 Console.ReadKey() 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
