How to customize workflow manager with .Net API

Hi

I am assigned to do some customization using .net API in Workflow manager.
Such as creating custom AOI(area of interest) selection tool that helps in ours own requirement.

I did not find any help in building add-in with .net API.
Even I cannot find the assembly that contain the Classes/Interfaces for Workflow manager.

Can someone help me ?

Thanks in Advance

Regards
Naym