Exactly what you need, I wrote it, along with the C# library :
Without throwing flower at myself, this is both the best lib and the best book you can get about bitcoin programming.
People have been developing C# bitcoin libraries for a while now, the real question is why would you use C#? You would probably want to employ encrypted storage of wallets and use Javascript on the client side to manage his funds.
Perhaps you want to have a balance on your website and have customers being able to deposit, which would be a whole other issue.
Because not everybody is developing client side wallet. The tip of the iceberg that the user see is less than 15% of the code which make any service run.
And NBitcoin is the only real library for Bitcoin the rest, BitcoinJ included are much less complete and tested.
But I would argue that C# is a good candidate for client wallet as well though, the code run on all plateform, without browser incompatibilities lurking around.