Joh*_*ger 8 c# exception covariance
在我的IRC应用程序中,应用程序从IRC服务器接收内容.内容被发送到工厂并且工厂吐出IMessage可以由应用程序的表示层使用的对象.的IMessage接口和单一的实现如下所示.
public interface IMessage
{
object GetContent();
}
public interface IMessage<out TContent> : IMessage where TContent : class
{
TContent Content { get; }
}
public class ServerMessage : IMessage<string>
{
public ServerMessage(string content)
{
this.Content = content;
}
public string Content { get; private set; }
public object GetContent()
{
return this.Content;
}
}
要接收IMessage对象,表示层将订阅在我的域层中发布的通知.通知系统将订阅者集合迭代到指定的IMessage实现,并向订阅者发出回调方法.
public interface ISubscription
{
void Unsubscribe();
}
public interface INotification<TMessageType> : ISubscription where TMessageType : class, IMessage
{
void Register(Action<TMessageType, ISubscription> callback);
void ProcessMessage(TMessageType message);
}
internal class Notification<TMessage> : INotification<TMessage> where TMessage : class, IMessage
{
private Action<TMessage, ISubscription> callback;
public void Register(Action<TMessage, ISubscription> callbackMethod)
{
this.callback = callbackMethod;
}
public void Unsubscribe()
{
this.callback = null;
}
public void ProcessMessage(TMessage message)
{
this.callback(message, this);
}
}
public class NotificationManager
{
private ConcurrentDictionary<Type, List<ISubscription>> listeners =
new ConcurrentDictionary<Type, List<ISubscription>>();
public ISubscription Subscribe<TMessageType>(Action<TMessageType, ISubscription> callback) where TMessageType : class, IMessage
{
Type messageType = typeof(TMessageType);
// Create our key if it doesn't exist along with an empty collection as the value.
if (!listeners.ContainsKey(messageType))
{
listeners.TryAdd(messageType, new List<ISubscription>());
}
// Add our notification to our listener collection so we can publish to it later, then return it.
var handler = new Notification<TMessageType>();
handler.Register(callback);
List<ISubscription> subscribers = listeners[messageType];
lock (subscribers)
{
subscribers.Add(handler);
}
return handler;
}
public void Publish<T>(T message) where T : class, IMessage
{
Type messageType = message.GetType();
if (!listeners.ContainsKey(messageType))
{
return;
}
// Exception is thrown here due to variance issues.
foreach (INotification<T> handler in listeners[messageType])
{
handler.ProcessMessage(message);
}
}
}
为了演示上述代码的工作原理,我有一个简单的Console应用程序,它可以订阅上述ServerMessage类型的通知.控制台应用程序首先通过将ServerMessage对象Publish<T>直接传递给方法来发布.这没有任何问题.
第二个示例让app使用工厂方法创建IMessage实例.然后将IMessage实例传递给Publish<T>方法,导致我的方差问题抛出InvalidCastException.
class Program
{
static void Main(string[] args)
{
var notificationManager = new NotificationManager();
ISubscription subscription = notificationManager.Subscribe<ServerMessage>(
(message, sub) => Console.WriteLine(message.Content));
notificationManager.Publish(new ServerMessage("This works"));
IMessage newMessage = MessageFactoryMethod("This throws exception");
notificationManager.Publish(newMessage);
Console.ReadKey();
}
private static IMessage MessageFactoryMethod(string content)
{
return new ServerMessage(content);
}
}
该异常声明我无法将INotification<IMessage>(发布方法理解发布的消息是什么)转换为INotification<ServerMessage>.
我试图将INotification接口标记为逆变,INotification<in TMessageType>但不能这样做,因为我正在使用TMessageType该Register方法的回调作为参数.我应该将接口分成两个独立的接口吗？一个可以注册,一个可以消费？这是最好的选择吗？
任何额外的帮助都会很棒.
我通过添加另一个间接级别解决了这个问题。遵循@PeterDuniho 的建议后，我将INotification<TMessage>界面分为两个单独的界面。通过添加新INotificationProcessor接口，我可以将侦听器集合从 an 更改ISubscription为 an INotificationProcessor，然后将侦听器集合作为 INotificationProcessor 进行迭代。
public interface ISubscription
{
void Unsubscribe();
}
public interface INotification<TMessageType> : ISubscription where TMessageType : class, IMessage
{
void Register(Action<TMessageType, ISubscription> callback);
}
public interface INotificationProcessor
{
void ProcessMessage(IMessage message);
}
该INotificationProcessor实现同时实现INotificationProcessor和INotification<TMessageType>。这允许Notification下面的类将提供的 IMessage 转换为适当的通用类型以进行发布。
internal class Notification<TMessage> : INotificationProcessor, INotification<TMessage> where TMessage : class, IMessage
{
private Action<TMessage, ISubscription> callback;
public void Register(Action<TMessage, ISubscription> callbackMethod)
{
this.callback = callbackMethod;
}
public void Unsubscribe()
{
this.callback = null;
}
public void ProcessMessage(IMessage message)
{
// I can now cast my IMessage to T internally. This lets
// subscribers use this and not worry about handling the cast themselves.
this.callback(message as TMessage, this);
}
}
My现在可以保存类型NotificationManager集合，而不是调用该方法，无论传入的内容是 an还是 a 。INotificationProcessorISubscriptionProcessMessage(IMessage)IMessageServerMessage
public class NotificationManager
{
private ConcurrentDictionary<Type, List<INotificationProcessor>> listeners =
new ConcurrentDictionary<Type, List<INotificationProcessor>>();
public ISubscription Subscribe<TMessageType>(Action<TMessageType, ISubscription> callback) where TMessageType : class, IMessage
{
Type messageType = typeof(TMessageType);
// Create our key if it doesn't exist along with an empty collection as the value.
if (!listeners.ContainsKey(messageType))
{
listeners.TryAdd(messageType, new List<INotificationProcessor>());
}
// Add our notification to our listener collection so we can publish to it later, then return it.
var handler = new Notification<TMessageType>();
handler.Register(callback);
List<INotificationProcessor> subscribers = listeners[messageType];
lock (subscribers)
{
subscribers.Add(handler);
}
return handler;
}
public void Publish<T>(T message) where T : class, IMessage
{
Type messageType = message.GetType();
if (!listeners.ContainsKey(messageType))
{
return;
}
// Exception is thrown here due to variance issues.
foreach (INotificationProcessor handler in listeners[messageType])
{
handler.ProcessMessage(message);
}
}
}
原始应用程序示例现在可以正常运行。
class Program
{
static void Main(string[] args)
{
var notificationManager = new NotificationManager();
ISubscription subscription = notificationManager.Subscribe<ServerMessage>(
(message, sub) => Console.WriteLine(message.Content));
notificationManager.Publish(new ServerMessage("This works"));
IMessage newMessage = MessageFactoryMethod("This works without issue.");
notificationManager.Publish(newMessage);
Console.ReadKey();
}
private static IMessage MessageFactoryMethod(string content)
{
return new ServerMessage(content);
}
}
感谢大家的帮助。