Question

假设我有以下界面：

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

我有一个实现此接口的抽象类：

 public abstract class AMessageProcessor<T> : IMessageProcessor<T> where T : BaseMessage {

        protected Controller Controller { get; private set; }

        public AMessageProcessor(Controller controller) {
            Controller = controller;
        }

        public abstract void Process(T msg);
    }

然后我有一条消息：

 public class RRMessage : BaseMessage {
  ...
 }

然后我有一个实现：

public class RRMessageProcessor : AMessageProcessor<RRMessage> {
      public RRMessageProcessor(Controller controller) : base(controller) {}

      public override void Process(RRMessage msg) {

            //do something here
        }
    }

现在在另一个类中，我想为不同的消息列出这些处理器：

 public readonly List<AMessageProcessor<BaseMessage>> AvailableMessageProcessors;

        public MessageProcessingStrategy(Controller controller) {
            AvailableMessageProcessors = new List<AMessageProcessor<BaseMessage>>();

            /* ----- ERROR HAPPENS AT THIS LINE ------ */
            AvailableMessageProcessors.Add(new RRMessageProcessor(controller));

        }

我收到了这个错误：

错误CS1503参数1：无法转换为＆＃39; RRMessageProcessor＆＃39;至＆＃39;＆AMessageProcessor LT; BaseMessage＆GT;＆＃39;

似乎转换应该有效...为什么它无法转换？我怎样才能让它发挥作用？

Answer 1

我的其他答案（删除）中有关于参数协变类型的问题，但这样的方法可以解决您的问题：

定义BaseMessageProcessor类（可能是AMessageProcessor），如下所示：

public abstract class BaseMessageProcessor
{
    protected Controller Controller { get; private set; }

    public BaseMessageProcessor(Controller controller)
    {
        Controller = controller;
    }

    public void Process<T>(T msg) where T : BaseMessage
    {
        if (this is IMessageProcessor<T>)
            (this as IMessageProcessor<T>).Process(msg);

        throw new NotSupportedException();
    }
}

定义接口IMessageProcessorOf<T>：

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

定义继承BaseMessageProcessor并实现（明确）IMessageProcessorOf<T>的具体处理器：

public class RRMessageProcessor : BaseMessageProcessor, IMessageProcessorOf<RRMessage>
{
    public RRMessageProcessor(Controller controller) : base(controller) { }

    void IMessageProcessor<RRMessage>.Process(RRMessage msg)
    {
        ...
    }
}

此解决方案允许您使用AvailableMessageProcessors：

public List<BaseMessageProcessor> AvailableMessageProcessors;

...

AvailableMessageProcessors = new List<BaseMessageProcessor>();
AvailableMessageProcessors.Add(new RRMessageProcessor(controller));

因此，如果您有2种邮件类型，例如RRMessage和SSMessage，则可以定义一个MultiMessageProcessor：

public class MultiMessageProcessor : BaseMessageProcessor, IMessageProcessorOf<RRMessage>, IMessageProcessorOf<SSMessage>
{
    public MultiMessageProcessor(Controller controller) : base(controller) { }

    void IMessageProcessorOf<RRMessage>.Process(RRMessage msg)
    {
        ...
    }

    void IMessageProcessorOf<SSMessage>.Process(SSMessage msg)
    {
        ...
    }
}

Process()方法的调用将通过BaseMessageProcessor.Process<>：

multiProcessor.Process(new RRMessage());
multiProcessor.Process(new SSMessage());

或者只使用RRMessageProcessor并使用与之前相同的想法定义SSMessageProcessor。

Answer 2

这不是最干净/最漂亮的方式，但似乎有效。这是我在 AMessageProcessor 中更改的内容：

public abstract class AMessageProcessor : IMessageProcessor<BaseMessage>

        protected Controller Controller { get; private set; }

        public AMessageProcessor(Controller controller) {
            Controller = controller;
        }

        /* ----- REALLY DON'T WANT TO OVERRIDE THIS METHOD EVERYWHERE --- */
        public abstract void Process(BaseMessage msg);
    }

然后将 RRMessageProcessor 更改为：

public class RRMessageProcessor : AMessageProcessor, IMessageProcessor<RRMessage> {
      public RRMessageProcessor(Controller controller) : base(controller) {}

      /* ----- REALLY DON'T WANT TO OVERRIDE THIS METHOD LIKE THIS EVERYWHERE --- */
      public override void Process(BaseMessage msg) {
            Process(msg as RRMessage);
        }

      public void Process(RRMessage msg) {

            //do something here
        }
    }

Answer 3

在您的通用类型上，您需要将类型定义为协变。见https://msdn.microsoft.com/en-us/library/dd997386.aspx

这应该有效

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

Answer 4

您可以通过重新定义

轻松解决问题

public readonly List<AMessageProcessor<BaseMessage>> AvailableMessageProcessors;

到

public readonly List<RRMessageProcessor> AvailableMessageProcessors;

因此，您不会遇到问题并且您正在使用自定义对象

希望能帮到你！

无法从Abstract <generic>转换为Concrete：Abstract <concrete：generic>

4 个答案: