public interface IBrand { }

    public class Bmw : IBrand { }

    public interface ICar<out TBrand> where TBrand : IBrand
    {
        void StartEngine();
        //void Method(TBrand brand);
    }

    public abstract class Car<TBrand> : ICar<TBrand> where TBrand : IBrand
    {
        public void StartEngine() { }
    }

    public class Suv1 : Car<Bmw> { }
    public class Suv2 : ICar<Bmw>
    {
        public void StartEngine() { }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Car<IBrand> bmwSuv1 = new Suv1(); //不行
            Car<IBrand> bmwSuv2 = new Suv2(); //不行

            ICar<IBrand> bmwSuv3 = new Suv1(); //可以, 但如果含有void Method(TBrand brand)方法，则会要求TBrand支持逆变，就又不行了
            ICar<IBrand> bmwSuv4 = new Suv2(); //同上

            // 想要的效果
            var cars = new List<ICar<IBrand>>();
            cars.Add(bmwSuv1);
            cars.Add(bmwSuv2);
            cars.Add(bmwSuv3);
            cars.Add(bmwSuv4);

            foreach (var car in cars)
            {
                car.StartEngine();
            }
        }
    }