Sometimes in VbClassic you want to get hold of an object but not keep a reference to it. 

An example would be a Parent property in a collection item. If you do it by holding a reference to the parent in each member of the collection, you end up with circular referenes.

Instead you can hold a Proxy to the parent object.

http://www.vb-faq.com/Articles/Hughson/proxyobjects.asp

This link seems to be dead, here is some example code 

You will need the Child, the Parent, and the Proxy Object

 Parent''''''Proxy Object
 --------------------------------
 Option Explicit
 Public Event Get''''''Ref(ByRef RHS As My''''''Parent)
 
 Public Function GetParent() As My''''''Parent
    Dim Ref As My''''''Parent
    Raise''''''Event Get''''''Ref(Ref)
    Set Get''''''Parent = Ref
 End Function
 
 Child Object
 ----------------------
 ..
 Private aParent as Parent''''''Proxy
 ..
 Public Property Get Parent() as My''''''Parent
    Set Parent = aParent.Get''''''Parent
 End Property
 
 Public Property Set Parent(RHS as My''''''Parent)
    Set aParent as My''''''Parent.Proxy
 End Property
 
 Parent Object
 ----------------------------
 
 Private WithEvents priParentProxy as Parent''''''Proxy
 
 Private Sub Class_Initialize()
  ..
  Set priParentProxy = New Parent''''''Proxy
  ..
 End Sub
 
 Friend Property Get Proxy() As Parent''''''Proxy
    Set Proxy = priParentProxy
 End Property
 
 Private Sub priParentProxy_GetRef(RHS As My''''''Parent)
    Set RHS = Me
 End Sub

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Occasionally, you may still be forced to work in Office 97, in which case, you can't define custom events.  In these cases, you have 2 alternatives that I know of:

* Have a single container object around one or more objects in your cyclic graph that is not, itself part of a cycle.  When this object goes out of scope, it calls methods of the objects it contains to tell them to drop their references to other objects in the network, so cyclical references are broken.
* Use a non-visible Form object as an event messenger.  You can force the Change event handler of a control on the form to fire by changing its Text property (works in MicrosoftAccess anyway).

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''One question: When can it happen that the proxied object is released, but you still try to reference it through the proxy? Without this knowledge, I am not certain that I dare to use this pattern.''
-- ThomasEyde

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CategoryVbClassic