泛型类型中的C#协方差

public class Animal { }
public class Dog : Animal { }
public class Cat : Animal { }

// 使用协方差的示例
IEnumerable<Dog> dogs = new List<Dog> { new Dog(), new Dog() };
IEnumerable<Animal> animals = dogs; // 这里使用了协方差

public interface ICovariant<out T>
{
    T GetItem();
}

public class CovariantClass<T> : ICovariant<T>
{
    private T _item;

    public CovariantClass(T item)
    {
        _item = item;
    }

    public T GetItem()
    {
        return _item;
    }
}

public class Program
{
    public static void Main()
    {
        ICovariant<Dog> dogCovariant = new CovariantClass<Dog>(new Dog());
        ICovariant<Animal> animalCovariant = dogCovariant; // 这里使用了协方差

        Animal animal = animalCovariant.GetItem();
        Console.WriteLine(animal.GetType().Name); // 输出: Dog
    }
}