Some time ago I released unfinished gamemode written in C# using plugin SampSharp.
You can download it for free to learn, it is a great example - what I did is login and registration system trought MySql and basic player spawn.

Download basic gamemode here:
https://github.com/Hy4e/SampSharp-GameMode
SampSharp site - write your SA:MP gamemode in modern language C#:
https://sampsharp.net/
How to setup project for your server?
https://sampsharp.net/getting-started

Good luck and feel free to post here if you face any issue!