我会让代码说话:
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:TFSFACEResearchAreaArgonServiceBusSpikeArgon_Service_Bus_Spike_v2Argon.ServiceLayertest.cs 31 13 Argon.ServiceGateway
// 2: Argument 1: cannot convert from 'test.ThingServiceInstance<test.MyThing>' to 'test.IThingServiceInstance<test.IThing>' C:TFSFACEResearchAreaArgonServiceBusSpikeArgon_Service_Bus_Spike_v2Argon.ServiceLayertest.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
而MyThing
是IThing
,为什么我不能将ThingServiceInstance<MyThing
>添加到IThingServiceInstance<IThing>
集合中?
我该怎么做才能编译此代码?
ThingServiceInstance<MyThing>
不是IThingServiceInstance<IMyThing>
的子类型,因为IThingServiceInstance<T>
的类型参数<T>
是不变的。
如果你想使ThingServiceInstance<MyThing>
成为IThingServiceInstance<IMyThing>
的子类型,那么T
必须是协变的。在 C# 中,可以通过声明IThingServiceInstance<T>
来实现:
public interface IThingServiceInstance<out T>
编辑然而,这意味着ThingServiceInstance<T>
只能返回 T 的实例,但永远不能将它们作为方法参数(因此使用"out"表示法)。
编辑2
这就是代码未编译的要点。如前所述,由于您的ThingServiceInstance<T>
公开了一个IThingRepository<T>
属性,因此它也必须是协变的,如下所示:
public interface IThingRepository<out T> where T : class, IThing { }
如下所示,您的属性必须是仅获取的(请记住,您只能返回 T
或 U<T>
的实例)。
如果你声明了两个接口 coariant,并从 IThingServiceInstance
中删除 setter ,你可以让它编译。
当然,您无法更改第三方界面,因此这并没有真正的帮助。
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>
没有声明T
协变,则
T : A
你没有得到
IThingRepository<T> : IThingRespository<A>
所以你不能有
IThingServiceInstance<T> : IThingServiceInstance<A>
因为 getter 返回的类型不"兼容"。