具有继承的嵌套通用接口

Question

定义：

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>> { }

代码：

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

为什么h1和h2都会导致NULL？

根据[HandlerA]的定义，它应该能够处理任何IEvent [IRecord]类型。

Answer 1

您应该将covariance更改为IHandler界面中的contravariance，它会起作用：

public interface IHandler<in T> { }

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