Re:Unreal Engine 4 is now available to everyone for free
by WillPostForFood on Monday March 02, 2015 @07:55PM
(#9135486)
To redistribute code written with Mono for Unreal Engine, you must have a commercial license to the Mono runtime. These licenses are available from Xamarin for Mac, Android and iOS online, and you can request Windows licenses through support.
https://mono-ue.github.io/about.html
If you want to code in C# it is the primary language for Unity, and a commercial third party unsupported add on for UE4.