[C#] Adding "partial" classes to the solution at runtime

Hi every one
 
I'll try to keep this short, my problem is i'm making some kind of a "game" (maybe more of an engine?) and the whole idea in what i try to make is that i have made everything ready for others to do, in other words you make your own game but i have made some basic things ready to use, maybe a little like minecraft, where you have some basic stuff but can add mods to it easily.
 
what i'm thinking is to make some class partial, e.g. i have a input system and i wanna have my own stuff there with some basic movement, but i wanna let the "modders" have the ability to add their own inputs by using a partial class and "simply" load every mod into the solution and just extend the input system with the user made partial class.
 
now here is the problem i honestly have no clue how i can add this class into the solution at runtime and still make it possible for the program to find these classes and use them as if they're partial/extender to my main input class.
 
so is this possible and how? code simples and sources would be lovely.
 
or if there is a better way of doing this, how?
 
or if it isn't possible, is there any other way i can make it work just the same?
 
Thanks for reading i appreciate you time and help
 
- Jackie
 
EDIT:
 
fast update: it just came to my mind i could also just load an assembly, however i feel this might be much more advance and it would also require the player/"modder"/stuff to compile every thing to a dll or exe which i really just wanna try to avoid since i wanna keep it simple for the modder/player so he can go right ahead in notepad or something.
 
an other one also go abit advance since this will require to almost make a whole new language which i REALLY wanna avoid, i can go in on a ini file scanner maybe but please no more here
 
Edit:
 
Ps. im using mono development for this game so the solution should be multi platform usable if possible.