S&box is coded in C#. Under the hood, it uses the Source 2 engine (CS2, HL:Alyx, DOTA2) and some of its systems: rendering, resources, physics, and audio. When you create games and addons in s&box, you will be creating them in C#. Facepunch have developed a hotload system which is capable of compiling & hotloading your changes to code within a few milliseconds, which negates the need for a scripting language.