Switching List value inside the For_loop

Hi, does any one know how to switch the values between two list inside the for loop.
As the simplified for-loop below. I want to switch the value of ptList_in with ptList_out after calling the Sub1, so that I can use the output points as the input points.
I try to use "PtList_out = PtList_in", but it didn't work.
Does anyone know how to do that? Many thanks.

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Dim PtList_in as New List(of On3dPoint)
Dim PtList_out as New List(of On3dPoint)

For i = 0 To 10


Call Sub_Pt(PtList_in, PtList_out)

'switching the value of Pt_in and Pt_out here??


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