empty attributes in C#?

Question

hi there,
I'm trying to call a third-party DLL, written in VB, from an app written in C#.
One of the methods I need to call is in the format:
records.populate(ref clientID, ref datefrom, ref dateto, ref status)
    (where clientID and status are both integers and the other two are datetimes)
In the documentation for the library, it states that all the attributes are optional, giving the example:
records.populate(123,,,)
How can I emulate this code in C#?
I've tried the same syntax (won't compile with empty arguments); tried using null for each one (well, I can't do this because DateTime is non-nullable); tried using new int()/DateTime(), but this didn't return any data when it should have.
So how do I do it?
I have no idea how the populate() method is handling the arguments, since it's third-party.

Thanks for your help.