Sven, I am sorry to tell you that you are actually reinventing the wheel, the Windows Vista SDK already comes with a great example of how to wrap the new vista dialog APIs in .net, they are both callable from windows forms and WPF applications, it is located in:

Your Installation Driver:\Program Files\Microsoft SDKs\Windows\v6.0\Samples\CrossTechnologySamples\VistaBridge

Sheva