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Tip of the Day
Language: VB7
Expertise: beginner
Jun 29, 2002

Understanding passing arrays by reference

.NET arrays are object types, thus an array variable is actually a pointer to the object where data is actually stored. For this reason, when you pass an array to a procedure the pointer is passed and the called procedure is always able to modify the elements of the array, regardless of whether the array has been passed by reference or by value. Apparently, there is no difference between passing an array with ByVal or ByRef.

The only case when you see a difference is if the called procedure REDIMs the array. If the array has been passed by reference, the original array is modified; if the array has been passed by value, a new array is created and the original array isn't modified. Even more important, assignments to the elements of this new array don't affect the original array in any way. This code makes the concept clear:


Sub Main()
    ' create three identical arrays
    Dim a() As Integer = {0, 1, 2}
    Dim b() As Integer = {0, 1, 2}
    Dim c() As Integer = {0, 1, 2}
    ' pass them by value and by reference to a procedure
    DoIt(a, b, c)

    ' prove that assign to elements of arrays passed by value
    ' affect the original array
    Console.WriteLine(a(0))       ' => 999
    ' prove that ReDim has no effect on arrays passed by value
    Console.WriteLine(b.Length)   ' => 3
    ' prove that ReDim affect arrays passed by reference
    Console.WriteLine(c.Length)   ' => 101
End Sub

Sub DoIt(ByVal a() As Integer, ByVal b() As Integer, ByRef c() As Integer)
    ' prove that assign to elements of arrays passed by value
    ' affect the original array
    a(0) = 999
    ' prove that ReDim has no effect on arrays passed by value
    ReDim b(100)
    ' prove that ReDim affect arrays passed by reference
    ReDim c(100)
End Sub

Francesco Balena
 
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