DDE in C#

There is a specialized code to software called Zemax which allows DDE communication with it. Code is written in C and handles some DDE events and describes some global functions. It also allows to user upon receiving some event to run its own code. What needs to be done. This code should be re-written in C# or there should be provided a method describing how user can assemble his own code written in C# with this code in C (with example). ## Deliverables Code that needs to be re-written is provided. File "hello_world.c" demonstrates where user-defined code is being called. File "zclient.c" is the main code that needs to be re-written in C#