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Collection was modified; enumeration operation may not execute

This is a WCF service in a windows carrier. The method notify subscribers() is referred to as via the carrier on every occasion there's a statistics event (at random durations, but no longer very frequently - approximately 800 times in line with day).

Whilst a home windows paperwork purchaser subscribes, the subscriber identity is added to the subscriber's dictionary, and whilst the patron unsubscribes, it's far deleted from the dictionary. The mistake happens when (or after) a client unsubscribes. It seems that the next time the notify subscribers() approach is known as, the foreach() loop fails with the mistake in the concerned line. The approach writes the mistake into the utility log as shown in the code below. When a debugger is attached and a purchaser unsubscribes, the code executes fine.

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);

        }

    }

}