You should be more specific;
connecting AS400 with C# is not enough.
- which kind of connection?
- what is the target of your application?
- what have you tried?
- which kind of problem have you encountered?
Here you wouldn't find anyone that will give you a complete solution and source code for your problem; but if you post detailed informations and some code snippet about what is not working properly, everyone will be happy to give you all the possible help.
Cheers,
Sauro