Hi Niwatchai,
if you feel like it, you can modify Java libRTI codebase/generator to generate C# libRTI. That could get you at least some functionality. If you write more about the RTI features your federate might need I may be more helpful.
Porting Java LibRTI to C# should be pretty straightforward. Java and C# are very similar and basically the only parts you would have to rewrite from scratch are encoding and socket connection. Other than that it is more about search & replace.
Anyway, second approach is to link native code as Eric wrote. I have played with it some time ago and it was not as painful as one would expect. As far as I understand, similarly to the first approach you would have to implement your own encoding helpers.
Andrej
PS: I am not sure if it helps but I would expect both approaches to take 2-3 weekends of systematical work for person familiar with CERTI.
Andrej Pancik
( +44 782 760 9197