My advice, as a professional game developer, is "don't write code unless you need to."
If you must write C# you should take a look at Unity, but honestly you would be remiss if you overlooked Unreal 4. It is the newest version of the Unreal Engine, and it includes most of the most important features of a game; behavior trees, navigation, user interface, physically-based rendering, animation state machines, multiplayer replication, etc. You can do most of what you want to in Blueprint (a visual scripting language), and if you really need to, you can edit the source code (the license is a full-source license) and make what enhancements you need in C++.
Writing most of this shit from scratch will take you years, just get down to the actual MAKING of the game, and use someone else's engine. The terms are fair ($20/mo, 5% gross revenue for PC platforms and mobile), and the engine is extremely well curated.