Please could you explain in detail what you want?
You can do in UE4 **anything ** you want in Blueprints. For the very few moments when you need super optimizations (speed optimization of a Blueprint, making an addon or making your super cool game/app code) make a freaking effort and do it in C++.
Why you need C#??? For what exactly What do you want to achieve besides adding a .net layer full of bindings? Please en-light me! =))