Is there any way to pack my functions or build a dll ?

So I have a collection of functions (lets call them commons), and I normally always copy paste them into the scripts I use them (I use them in every game, they are stuff like, scene manager, game save/load and stuff) and I wanted to ask if there way a way to pack them somehow and include them in my scripts like libraries or header files.

And if there was a way to build a dll it would be even better! (I work as a freelancer and when my client asks for the source so he can do variable tweaks, It would be pretty good if I could protect my code that way)

This could be the wrong section but thought since its scripting some of you might know how to do it.

Sure, create a new project in monodevelop, add refrence to the UnityEngine.dll then create your code, etc. Then simply build it as a dll.

Project type should be class library