收藏被修改；枚举操作可能不会执行 [英] Collection was modified; enumeration operation may not execute

问题描述

我无法深究这个错误的根源，因为当附加调试器时，它似乎没有发生.

I can't get to the bottom of this error, because when the debugger is attached, it does not seem to occur.

集合被修改；枚举操作可能不会执行

Collection was modified; enumeration operation may not execute

下面是代码.

这是一个 Windows 服务中的 WCF 服务器.只要有数据事件(随机间隔，但不是很频繁 - 每天大约 800 次)，服务就会调用方法 NotifySubscribers().

This is a WCF server in a Windows service. The method NotifySubscribers() is called by the service whenever there is a data event (at random intervals, but not very often - about 800 times per day).

当 Windows 窗体客户端订阅时，订阅者 ID 被添加到订阅者字典中，当客户端取消订阅时，它会从字典中删除.当(或之后)客户端取消订阅时会发生错误.似乎下次调用 NotifySubscribers() 方法时，foreach() 循环失败并在主题行中出现错误.该方法将错误写入应用程序日志，如下面的代码所示.当附加调试器并且客户端取消订阅时，代码可以正常执行.

When a Windows Forms client subscribes, the subscriber ID is added to the subscribers dictionary, and when the client unsubscribes, it is deleted from the dictionary. The error happens when (or after) a client unsubscribes. It appears that the next time the NotifySubscribers() method is called, the foreach() loop fails with the error in the subject line. The method writes the error into the application log as shown in the code below. When a debugger is attached and a client unsubscribes, the code executes fine.

您是否发现此代码有问题?我需要使字典线程安全吗?

Do you see a problem with this code? Do I need to make the dictionary thread-safe?

[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class SubscriptionServer : ISubscriptionServer
{
    private static IDictionary<Guid, Subscriber> subscribers;

    public SubscriptionServer()
    {            
        subscribers = new Dictionary<Guid, Subscriber>();
    }

    public void NotifySubscribers(DataRecord sr)
    {
        foreach(Subscriber s in subscribers.Values)
        {
            try
            {
                s.Callback.SignalData(sr);
            }
            catch (Exception e)
            {
                DCS.WriteToApplicationLog(e.Message, 
                  System.Diagnostics.EventLogEntryType.Error);

                UnsubscribeEvent(s.ClientId);
            }
        }
    }
    
    public Guid SubscribeEvent(string clientDescription)
    {
        Subscriber subscriber = new Subscriber();
        subscriber.Callback = OperationContext.Current.
                GetCallbackChannel<IDCSCallback>();

        subscribers.Add(subscriber.ClientId, subscriber);
        
        return subscriber.ClientId;
    }

    public void UnsubscribeEvent(Guid clientId)
    {
        try
        {
            subscribers.Remove(clientId);
        }
        catch(Exception e)
        {
            System.Diagnostics.Debug.WriteLine("Unsubscribe Error " + 
                    e.Message);
        }
    }
}

推荐答案

可能发生的事情是 SignalData 在循环期间在幕后间接更改订阅者字典并导致该消息.您可以通过更改来验证这一点

What's likely happening is that SignalData is indirectly changing the subscribers dictionary under the hood during the loop and leading to that message. You can verify this by changing

foreach(Subscriber s in subscribers.Values)

到

foreach(Subscriber s in subscribers.Values.ToList())

如果我是对的，问题就会消失.

If I'm right, the problem will disappear.

调用 subscribers.Values.ToList() 将 subscribers.Values 的值复制到 foreach 开头的单独列表中.没有其他人可以访问这个列表(它甚至没有变量名！)，所以没有什么可以在循环内修改它.

Calling subscribers.Values.ToList() copies the values of subscribers.Values to a separate list at the start of the foreach. Nothing else has access to this list (it doesn't even have a variable name!), so nothing can modify it inside the loop.

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