World

public class Tank : Unit
{
    public void DoSomething() => World.CreateTank();
}

public class Unit
{
    protected readonly World World;
    public Unit(World world) => this.World = world;
}


public class World
{
    public World()
    {
        _units = new Units();
        _unitFactory = new UnitFactory(this);
    }

    public void CreateTank() => _units.Add(_unitFactory.CreateTank());

    UnitFactory _unitFactory;
    Units _units;

}


public class UnitFactory
{
    public void Create(string name) => new Unit(_world);
    public void CreateTank() => new Tank(_world);

    readonly World _world;
    public UnitFactory(World world) => this._world = world;
}