似乎无法使通用协方差/协方差工作

Question

假设我有以下代码：

public interface IBaseMessage { }

public interface IMessageProcessor<in T> where T : IBaseMessage {
            void Process(T msg);
        }

public class RRMessage : IBaseMessage {
          //something here
        }

public class BaseMessageProcessor {
         //something here
        }

public class RRMessageProcessor : BaseMessageProcessor, IMessageProcessor<RRMessage> {
            public void Process(RRMessage msg) {
                Console.WriteLine("Processed RRMessage");
            }
        }

public Dictionary<Type, IMessageProcessor<IBaseMessage>> MessageProcessors = new Dictionary<Type, IMessageProcessor<IBaseMessage>>();

[Test]
public void Test1() {
     var msgProcessor = new RRMessageProcessor();
     MessageProcessors.Add(typeof(RRMessage), msgProcessor);
     }

我为接口IMessageProcessor启用了反向方差。为什么MessageProcessors.Add(typeof(RRMessage), msgProcessor);会导致编译时错误：

论点2:不能从“RRMessageProcessor”转换为“IMessageProcessor”

似乎它应该能够转换，因为RRMessageProcessor:IMessageProcessor<RRMessage:IBaseMessage>>如何才能让它工作呢？

Answer 1

IMessageProcessor<in T>在T中是逆变的，而不是协变的。这意味着允许采取以下措施：

class RRSubtype : RRMessage {}
IMessageProcessor<RRSubtype> p = new RRMessageProcessor();

您试图做的是静态不安全的，因为它允许您执行以下操作：

class NotRRMessage : IBaseMessage { }
IMessageProcessor<IBaseMessage> p = new RRMessageProcessor();
p.Process(new NotRRMessage());

因此，您需要动态地维护安全性，这似乎是在尝试使用字典Type -> Processor。因此，您可以创建不安全的包装器类型：

public class ProcessorWrapper<T> : IMessageProcessor<IBaseMessage> {
    private readonly IMessageProcessor<T> inner;
    public ProcessorWrapper(IMessageProcessor<T> inner) { this.inner = inner; }

    public void Process(IBaseMessage msg)
    {
        if(msg is T) { inner.Process((T)msg); }
        else throw new ArgumentException("Invalid message type");
    }
}

然后，您可以使用这些包装器构造您的列表，同时维护您想要的内部处理器。

public Dictionary<Type, IMessageProcessor<IBaseMessage>> MessageProcessors = new Dictionary<Type, IMessageProcessor<IBaseMessage>>();
MessageProcessors.Add(new ProcessorWrapper<RRMessage>(new RRMessageProcessor());