Thanks to all!
I am almost sure that I have the correct 4.5 assemlies in the references because it shows me the wanted properties. But if I compile it fails. So I think that Powerbuilder uses the wrong compiler.
I have now created an assembly outside from Powerbuilder and compiled it with the 4.5 compiler. I can this use within my application.
Would be nice if Powerbuilder would be really 4.5 compatible.
Marks suggestion would be great.
Regards
René