C# DLL cannot affect value of a number passed by reference from a VB6 application

Question

I have a legacy VB6 application that calls a VB6 DLL, and I am trying to port the VB6 DLL to C# without yet touching the main VB6 application code. The old VB6 DLL had one

Answer 1

dll.Increment( num )

Because you are using parentheses, the value is forcibly passed by value, not by reference (the compiler creates a temporary copy and passes that by reference).

Remove the parentheses:

dll.Increment num

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EDIT: A more complete explanation by MarkJ.