.NET：为什么需要逆变和协变

为啥需要协变和逆变？

一个小例子

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace InOutStudy
{
class Program
{
static void Main(string[] args)
{
IEnumerable<Base> list = new List<Child> { new Child() };

Console.WriteLine((typeof(ICollection<Base>)).IsAssignableFrom((typeof(List<Child>))));

IWorker<Child> woker = new Worker<Base>();

Console.WriteLine((typeof(IWorker<Child>)).IsAssignableFrom((typeof(Worker<Base>))));
}

class Base { }

class Child : Base { }
}

interface IWorker<in T>
{
void Dot(T t);
}

class Worker<T> : IWorker<T>
{
public void Dot(T t)
{
Console.WriteLine(t);
}
}
}

备注