Cloning a Collection - vba

Question

I have two collections - collection1 and collection2
collection1 has a number of class objects in it and I am trying to fill collection2 with copies of the same objects using the following command:
Set collection2 = collection1
This doesn't give me the desired result though, because when I use
collection2.Remove 1
It removes the object at index 1 from both collections.
Below is the full code, which I hoped would output 10 objects in collection1 after removing one from collection2
Sub test()
Dim collection1 As Collection
Dim collection2 As Collection
Dim testObj As Worksheet
Dim i As Integer
Set collection1 = New Collection
Set collection2 = New Collection
For i = 1 To 10
collection1.Add testObj
Next i
Set collection2 = collection1
collection2.Remove 1
Debug.Print collection1.Count
End Sub
I tried the code below and it works but I'm looking to avoid filling both collections one by one if possible:
...
For i = 1 To 10
collection1.Add testObj
collection2.Add testObj
Next i
...
The reason I'm not so keen on this option is because ultimately I intend to use multiple collections, manipulating them and taking copies at various points so I would end up with lots of for loops in my code, rather than just one.

Answer 1

I am trying to fill collection2 with copies of the same objects using the following command:
Set collection2 = collection1
Yet that's not what that command does. The Set keyword doesn't "copy objects", and doesn't automagically know (or even care) that it's dealing with a Collection object that contains items.
The Set keyword assigns a reference. Nothing more, nothing less.
So what Set collection2 = collection1 does, is, literally:
Take the pointer for the collection1 object, and replace whatever collection2 is referring to with that.
That Set instruction is effectively discarding the object you created in the first place, when you did this:
Set collection2 = New Collection
You're not "copying objects" or "filling collection2" - you're overwriting its object reference, making collection2 point to the same object as collection1.
And since both pointers point to the same object... removing an item using either Collection is indeed going to be removing it from "both" collections, ...because there's only one collection object involved.
The (undocumented?) ObjPtr keyword can help shed some light on what's happening, too:
Set collection1 = New Collection
Debug.Print "collection1: " & ObjPtr(collection1)
Set collection2 = New Collection
Debug.Print "collection2: " & ObjPtr(collection2)
'the debug output contains 2 different addresses at this point
Set collection2 = collection1
Debug.Print "collection1: " & ObjPtr(collection1)
Debug.Print "collection2: " & ObjPtr(collection2)
'now the debug output clearly shows that
'both collection1 and collection2 are pointing to the same object
If you need copies of a collection, you need a function that takes a collection and returns a brand new object:
Public Function CopyCollection(ByVal source As Collection) As Collection
Dim result As New Collection
Dim item As Variant
For Each item In source
result.Add item
Next
Set CopyCollection = result
End Function
As already noted in comments though, this will only work if your collections aren't keyed. Because of how limited a Collection is (you can't iterate its keys), you'll have to use a Scripting.Dictionary instead, if you need to clone a keyed collection.

Answer 2

First of all, you dont need to Dim collection1 As Collection and Set collection1 = New Collection. This can be replaced by Dim collection1 As New Collection
Now, what you are doing is making the collection2 to contain the collection1. Its like putting lots of caps on a box (collection1) and then putting this box inside another box (collection2).
The alternative is to either fill both of them at the same time or do a code to copy the contents like
for i = 1 to collection1.count
collection2.add collection1.item(i)
next i