无法从抽象<通用>转换为具体:抽象<具体:通用>

本文关键字：通用 抽象 具体 转换 | 更新日期: 2023-09-27 18:37:24

假设我有以下接口：

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));
        }

我收到此错误：

错误 CS1503 参数 1： 无法从"RRMessageProcessor" 转换为 'AMessageProcessor'

似乎转换应该有效...为什么它不能转换？我怎样才能让它工作？

无法从抽象<通用>转换为具体:抽象<具体:通用>

我在另一个答案（已删除）中遇到了关于参数协变类型的问题，但这样的方法可能会解决您的问题：

  1. 定义一个BaseMessageProcessor类（可以是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();
    }
}

  2. 定义接口IMessageProcessorOf<T>：

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

  3. 定义继承BaseMessageProcessor和实现（显式）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 种消息类型，例如 RRMessage 和 SSMessage ，您可以定义一个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)
    {
        ...
    }
}

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

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

或者只是使用RRMessageProcessor并使用与以前相同的想法定义SSMessageProcessor。

这不是最干净/最漂亮的方法，但它似乎有效。以下是我在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
        }
    }

在泛型类型上，需要将类型定义为协变。请参阅 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);
}

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，您没有转换问题，并且使用的是自定义对象

希望对您有所帮助！

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