>Is there a possibility to use optional parameters when defining a VB.NET function? If I add one additional parameter, wherever I have a call to that function, the compiler will immediately show the line as an error.

I think you may have some sort of problem with C# clients running your methods with optional parameters, since C# doesn't understand that. I'm not sure what sort of problem you may find since I've never tried that, but I think it worths keeping an eye on it. :)