似乎无法让通用 Covariance/Contravariance 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 启用了逆变。为什么MessageProcessors.Add(typeof(RRMessage), msgProcessor);会导致编译时错误：

Argument 2: cannot convert from 'RRMessageProcessor' to 'IMessageProcessor<IBaseMessage>'

它似乎应该能够转换，因为 RRMessageProcessor:IMessageProcessor<RRMessage:IBaseMessage>> 我怎样才能让它工作？

Answer 1

IMessageProcessor<in T>在T中是逆变的，不是协变的。这意味着允许以下内容：

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

您尝试做的事情在静态上是不安全的，因为它允许您：

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

所以你需要动态地维护安全，你似乎正在尝试使用字典 Type -> Processor。因此，您可以创建一个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建您的列表，同时维护您想要输入的内部处理器，例如

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

似乎无法让通用 Covariance/Contravariance 工作

Can't seem to get Generic Covariance/Contravariance to work

c#

generics

covariance

contravariance