Even with best will, it is impossible to give you the answer you want.
Quote:
I want to split an excel file into multiple sheets or multiple files.
This is the only part that explain what you want, but it is no way near something usable to make the program you want, so any work on the question is a waste of time.
For now, the best explanation you can do is publish the source code you have in VBA.
Quote:
I am totally blind about the logic
The logic of the C# will be exactly the same as VBA.
since you don't get the logic, I suspect you are not a programmer.
Programmers spend a awful lot of time studying to learn all necessary techniques and acquire the necessary knowledge. That is why people like you, that have no time or no <ill to learn programming, pay them to do the job.
Your question fit in category "do my job/homework for free", that why you will have greatest difficulties to find someone to it for free.
My best advice: hire a programmer.