c# – 似乎无法获得通用协方差/反协方差

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);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>>我怎样才能让它发挥作用？

解决方法:

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

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

你想要做的是静态不安全,因为它允许你这样做：

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

所以你需要动态地保持安全性,你似乎正在尝试使用字典Type – >处理器.因此,您可以创建一个不安全的包装类型：

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列表,同时保持内部处理器的输入,例如,

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

标签：c,covariance,contravariance,generics
来源： https://codeday.me/bug/20190702/1355339.html