具有繼承的嵌套泛型接口

Question

定義：

public interface IRecord { }
public interface IEvent<out T> { }
public interface IHandler<out T> { }

public class RecordA : IRecord { }
public class EventA : IEvent<RecordA> { }
public class HandlerA : IHandler<IEvent<IRecord>> { }

碼：

IHandler<EventA> h1 = new HandlerA() as IHandler<EventA>;
IHandler<IEvent<RecordA>> h2 = new HandlerA() as IHandler<IEvent<RecordA>>;

為什么h1和h2都為NULL？

基於[HandlerA]的定義，它應該能夠處理任何IEvent [IRecord]類型。

Answer 1

您應該改變協方差為逆變在IHandler界面，它會工作：

public interface IHandler<in T> { }

//It works
var h1 = new HandlerA() as IHandler<EventA>;
var h2 = new HandlerA() as IHandler<IEvent<RecordA>>;