Singleton.cs

using System.Collections.Generic;
using System.Linq;
using System.Numerics;

namespace NetCoreConsolePlayground
{
    class Entity
    {
        public Vector3 Postion;
        public Quaternion Rotation;
    }

    class Player : Entity
    {
        public string PlayerId;
        public IEnumerable<Monster> FindNearbyMonsters()
        {
            if (Game.Instance == null)
            {
                throw new Exception("There is no game!");
            }
            
            var allMonsters = Game.Instance.Monsters;
            return allMonsters.Where(m => Vector3.Distance(m.Postion, this.Postion) < 2).ToList();
        }
    }

    class Monster : Entity
    {
        public string MonsterType;
    }

    class Game
    {
        public Player Player;
        public IEnumerable<Monster> Monsters = new List<Monster>();

        public static Game Instance = null;

        public Game()
        {
            Instance = this;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Game game = new Game();
            List<Monster> monsters = new List<Monster>();
            monsters.Add(new Monster());
            monsters.Add(new Monster());
            monsters.Add(new Monster());
            monsters.Add(new Monster());
            game.Monsters = monsters;

            Player player = new Player();
            game.Player = player;

            var nearbyMonsters = player.FindNearbyMonsters();
        }
    }
}