How to Fix the 'Collection was Modified; Enumeration Operation May Not Execute' Error in WCF?

Troubleshooting the "Collection was Modified; Enumeration Operation May Not Execute" Error in WCF

The frustrating "Collection was modified; enumeration operation may not execute" error often disappears when debugging, making diagnosis difficult. This article examines a specific WCF server scenario and offers solutions.

The issue arises in a Windows service using a dictionary to track data event subscribers. Removing a client's ID (unsubscription) sometimes causes the NotifySubscribers method to throw this error.

The likely culprit is a callback, such as SignalData, modifying the subscriber dictionary during the NotifySubscribers loop. To resolve this, consider changing the foreach loop:

Original code:

foreach(Subscriber s in subscribers.Values)

Modified code:

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

By creating a copy of the subscribers.Values using ToList(), the enumeration operates on a snapshot of the dictionary's state at the loop's start. Any subsequent modifications to the original dictionary will not affect the enumeration, thus eliminating the error if the SignalData callback is the source of the problem. This isolates the loop from external changes, ensuring smooth execution.

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