Question

我会让代码进行讨论：

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IThingServiceInstance且MyThing是IThing，为什么我无法添加ThingServiceInstance<MyThing＆gt;收集到IThingServiceInstance<IThing>？

如何编译此代码？

Answer 1

ThingServiceInstance<MyThing> 不属于IThingServiceInstance<IMyThing>的子类型，因为IThingServiceInstance<T>在其类型参数<T>中是不变

如果您想ThingServiceInstance<MyThing> IThingServiceInstance<IMyThing>的子类型，那么T必须是协变的。在C＃中，您可以通过声明IThingServiceInstance<T>来实现这一点：

public interface IThingServiceInstance<out T>

修改然而，这意味着ThingServiceInstance<T>只能返回T的实例，但从不将它们作为方法参数（因此称为“out”符号）。

<强> EDIT2

这是您的代码未编译的原因。正如所指出的那样，由于您的ThingServiceInstance<T>公开了IThingRepository<T>属性，因此它也必须是协变的：

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

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

Answer 2

如果你声明两个接口coariant，你可以让它编译，并从IThingServiceInstance中删除setter。

当然，您无法更改第三方界面，因此这并没有真正帮助。

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

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

如果IThingRepository<T>未声明T协变，则

T : A

你没有得到

IThingRepository<T> : IThingRespository<A>

所以你不能拥有

IThingServiceInstance<T> : IThingServiceInstance<A>

因为getter返回的类型不是“兼容的”。

无法将通用接口的具体实例添加到泛型集合中

2 个答案: