How do I pass an SSRS parameter value to Business logic data method?

Let say for example I have an SSRS parameter called "Invoice ID:".

The user then types a value in this parameter box eg "INV200123" and hits view report.

At exactly this point, how would I go about retrieving this entered value in a data method?

I'm crudely imagining something like:

public static string GetParameter(string param_invoiceId)

{

param_invoiceId = [some code that grabs the parameter value]

}

Any ideas or pointers?

This is using SSRS 2008 and AX 2009 BTW.