Using a control as part of an exression

I am creating a procedure that can be used with both treeviews and
listboxes, and will take a control as one of the arguments. To save
myself having to create 2 overload procedures I would like to create 1
that recognises when it has a listbox, and when it has a treeview, so
that it can process the data appropriately. I have tried looking around
for answers without much luck. 

So... how do I use a control (such as a listbox) as an argument in an
expression. I.e.:
If control is a listbox then...

Thanks
Matt Bell
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