1. You can run the as400 procedures directly from your c# program.
For this you have to install IBM client.
2. as400 normally use DB2 database. you can directly connect DB2 database from you c# code.
For this you can directly use ADO.NET
3. You can write restful(AXIS 2) webservice and deploy in the as400. You can access that web service from your c# code.
For this you have to write Java code and install in the as400
4. Apart from this you can refer "cwbx.dll" and write as400 codes directly
Direction is important than speed
you can read about it more in following link
Editor, DotNetSpider MVM
Microsoft MVP 2014 [ASP.NET/IIS]