How to Prevent the 'Collection was modified; enumeration operation may not execute' Exception in WCF Services?

Addressing the "Collection Modified" Exception in WCF

This article examines the common "Collection was modified; enumeration operation may not execute" exception, frequently encountered when working with collections in multi-threaded environments, particularly within WCF services.

Scenario and Code Breakdown

The exception arises when a collection is iterated while simultaneously being modified. Imagine a WCF service using a dictionary to track subscribers. A method iterates through this dictionary to notify subscribers, but another process (perhaps within the same method) modifies the dictionary concurrently. This leads to the exception.

Root Cause Analysis

The problem stems from the concurrent modification of the subscriber dictionary during iteration. If a SignalData method, for instance, removes or updates a subscriber within the iteration loop, the collection's structure changes, causing the exception.

Solution: Create a Safe Copy

The solution lies in creating a copy of the collection before starting the iteration. Instead of directly iterating over the subscribers.Values, create a copy using ToList():

<code class="language-csharp">foreach (Subscriber s in subscribers.Values.ToList())</code>

This creates a snapshot of the collection's state at that moment. Modifications to the original subscribers dictionary will not affect this new list, preventing the exception.

Summary

The "Collection was modified" exception highlights the dangers of concurrent access and modification of collections. By using a copy of the collection for iteration, we ensure thread safety and prevent the exception, maintaining the integrity of the enumeration process.

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