c# - 无法将通用接口(interface)的具体实例添加到通用集合

Question

我会让代码来说话:

using System.Collections.Generic;

namespace test
{
    public interface IThing { } // can't change this - it's a 3rd party thing

    public interface IThingRepository<T> where T : class, IThing { } // can't change this - it's a 3rd party thing

    public interface IThingServiceInstance<T>
      where T : class, IThing
    {
        IThingRepository<T> Repository { get; set; }
    }

    public class ThingServiceInstance<T> : IThingServiceInstance<T> where T : class, IThing
    {
        public IThingRepository<T> Repository { get; set; }

    }

    public class MyThing : IThing
    {
    }

    class Test
    {
        public void DoStuff()
        {
            IList<IThingServiceInstance<IThing>> thingServiceInstances = new List<IThingServiceInstance<IThing>>();
            // the following line does not compile. Errors are:
            // 1: The best overloaded method match for 'System.Collections.Generic.ICollection<test.IThingServiceInstance<test.IThing>>.Add(test.IThingServiceInstance<test.IThing>)' has some invalid arguments    C:\TFS\FACE\ResearchArea\ArgonServiceBusSpike\Argon_Service_Bus_Spike_v2\Argon.ServiceLayer\test.cs 31  13  Argon.ServiceGateway
            // 2: Argument 1: cannot convert from 'test.ThingServiceInstance<test.MyThing>' to 'test.IThingServiceInstance<test.IThing>'    C:\TFS\FACE\ResearchArea\ArgonServiceBusSpike\Argon_Service_Bus_Spike_v2\Argon.ServiceLayer\test.cs 31  39  Argon.ServiceGateway
            // Why? ThingServiceInstance is an IThingServiceInstance and MyThing is an IThing
            thingServiceInstances.Add(new ThingServiceInstance<MyThing>());
        }
    }
}

如果ThingServiceInstance是一个 IThingServiceInstance和 MyThing是一个 IThing ，为什么我不能添加 ThingServiceInstance<MyThing > 到 IThingServiceInstance<IThing> 的集合？

我该怎么做才能编译此代码？

Answer 1

ThingServiceInstance<MyThing> 不是 IThingServiceInstance<IMyThing> 的子类型，因为 IThingServiceInstance<T>在其类型参数中不变 <T> .

如果你想制作ThingServiceInstance<MyThing> IThingServiceInstance<IMyThing> 的子类型, 然后 T必须是协变的。在 C# 中，您通过声明 IThingServiceInstance<T> 来做到这一点因此:

public interface IThingServiceInstance<out T>

编辑然而，这意味着 ThingServiceInstance<T>只能返回 T 的实例，但绝不能将它们作为方法参数(因此使用“out”表示法)。

编辑2

这就是您的代码无法编译的原因。正如所指出的，由于您的 ThingServiceInstance<T>暴露一个 IThingRepository<T>属性，它也必须像这样协变:

public interface IThingRepository<out T> where T : class, IThing { }

如下所示，您的属性必须是只读的(请记住，您只能返回 T 或 U<T> 的实例)。