Calling .Net code from Orchestration

Hello,

I have an orchestration that receives a message.  Once the orchestration 
receives a message, it is supposed to invoke our custom API and generate a

series of messages.

Does anybody have pointers to some tutorials for invoking C# code from an 
orchestration?  Are there any best practices or Dos and Don'ts for
invoking 
C# from Orchestrations?

Thanks,

Waqar Sadiq