Strange “type of”
I have a piece of code, as shown in the figure below.

Why isbill is “false” ? If the type of c is form, the above situation will occur; Not if the type of c is control.
If you look through the issues in this repo, you will find a few discussions about TypeOf, and how its behaviour is not always what people expect.
@kingtu
Could this be an issue about the debugger and the Watch window? I would suggest using the Quick Watch and the Immediate windows, and see what they display for TypeOf c Is IBill and isBill. You can also use them to confirm that c actually contains a reference to some object which implements IBill.
I suspect that the TypeOf operator is working as it should, but that the Watch window might not be getting properly updated (and hence making TypeOf c Is IBill and isBill seem out of sync).
Is this... using the full-width keywords of legend?