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05.06.2008 at 05:48PM PDT, ID: 23381362
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How to pass an object in an array by reference
Tags: Microsoft, VB.net
Hi Experts!
I have a structure in an array that holds instances of that structure.  For example:
Public structure MyStruct
     public i as integer
    public y as integer
End structure
public StructureArray as MyStruct()

I'm finding that when I pass a reference to an item in the array, say StructureArray(0) to a method,  even if the method's parameter is defined as ByRef, the method is operating on a copy of the structure and not the structure itself.
so this method won't really change the values of i/y in the Structure at StructureArray(0)
Sub MyMethod(byref s as MyStruct)
   s.i = 0
  s.y = 1
End sub

is it possible to accomplish this somehow?
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Question Stats
Zone: Programming
Question Asked By: sfun28
Solution Provided By: Idle_Mind
Participating Experts: 2
Solution Grade: A
Views: 7
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05.06.2008 at 06:18PM PDT, ID: 21512449

Rank: Wizard

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05.06.2008 at 06:19PM PDT, ID: 21512450

Rank: Genius

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05.06.2008 at 06:35PM PDT, ID: 21512519

Rank: Genius

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05.06.2008 at 07:20PM PDT, ID: 21512730

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05.06.2008 at 07:33PM PDT, ID: 21512783

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05.06.2008 at 06:18PM PDT, ID: 21512449

Rank: Wizard

check your code again.
its working as expected. if you param is defined as byref its changing the value of the item from the array. if byval its not changing. to confirm i tested here and the code is

    Public Structure MyStruct
        Public i As Integer
        Public y As Integer
    End Structure

    Sub MyMethod_Val(ByVal s As MyStruct)
        MsgBox(s.i)
        s.i = 0
        s.y = 1
        MsgBox(s.i)
    End Sub

    Sub MyMethod_Ref(ByRef s As MyStruct)
        MsgBox(s.i)
        s.i = 0
        s.y = 1
        MsgBox(s.i)
    End Sub

  Private Sub Button6_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button6.Click

        Dim StructureArray(1) As MyStruct

        StructureArray(0).i = 10
        StructureArray(0).y = 20

        MsgBox(StructureArray(0).i)

        MyMethod_val(StructureArray(0))

        MsgBox(StructureArray(0).i)

        MyMethod_ref(StructureArray(0))

        MsgBox(StructureArray(0).i)


    End Sub


 
05.06.2008 at 06:19PM PDT, ID: 21512450

Rank: Genius

Basically want you want can't be done using Structures as they are "value types".  Change to a Class to achieve the functionality you want...

Read up here:
http://msdn.microsoft.com/en-us/library/2hkbth2a(VS.71).aspx

    "When you assign one structure variable to another, or pass a structure instance to a procedure argument, the current values of all the variable members are copied to the new structure."
 
05.06.2008 at 06:35PM PDT, ID: 21512519

Rank: Genius

It seems I am WRONG...   =\
 
05.06.2008 at 07:20PM PDT, ID: 21512730
sorry - i forgot to mention that i'm looping through the array using a "for each" loop
so:
for each sa in StructureArray
MyMethod_ByRef(sa)
next

this is what seems to not be working.  so perhaps the for...each is creating a copy instead of referencing the object?
 
05.06.2008 at 07:33PM PDT, ID: 21512783

Rank: Genius

Do it this way instead:

    For i As Integer = 0 to StructureArray.GetUpperBound(0)
        MyMethod_ByRef(StructureArray(i))
    Next i
Accepted Solution
 
 
05.06.2008 at 07:39PM PDT, ID: 21512817
yes for each creates the copy

try like this

 For i As Integer = 0 To StructureArray.GetUpperBound(0)

            MsgBox(StructureArray(i).i)

            MyMethod(StructureArray(i))

            MsgBox(StructureArray(i).i)

        Next
 
 
 
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