Hi Panos...
What I know is that PBDOM is unsupported in .net targets... They recommend using .net's available classes. But maybe you could create a com object (of course, in an older version of PB, ex. 11.5.1), and use it in your c# service... I never tried, but it's the only solution I can think in that case... Anyway, the best solution would be coding all the service in c#.
Andreas.