>The outside application that uses this dll passes all parameters as XML string. The VFP dll then parses that XML, finds out which method was called and then passes all of them as parameters.

That sounds like my first suggestion. You already have the content in XML. You might just reroute it to your backend as is, and/or add what is missing.

>Since in this particular case we're close to the limits of VFP as number of parameters of a method, I think I have no other choice as to have that parameter to become a class property instead.

That would do as well.