Remove custom enumerator from ConcurrentBag #117353
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ConcurrentBag's snapshot enumeration semantics are obtained by creating an array of the contents and then enumerating that array. This was done with a custom enumerator out of an abundance of caution about maintaining semantics for operations with undefined behavior, e.g. using Current after MoveNext returns false. We've since stopped trying to maintain bug-for-bug compatibility in such undefined areas. As such, this just deletes the custom enumerator and uses Array's. This not only reduces code but has better performance characteristics (though the whole ToArray as part of enumeration already dominates the enumeration costs).
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Pull Request Overview
This PR removes the custom Enumerator class from ConcurrentBag<T> and switches its GetEnumerator implementation to use the array’s built-in enumerator, reducing code and improving performance.
- Swapped out
new Enumerator(ToArray())for((IEnumerable<T>)ToArray()).GetEnumerator() - Deleted the custom
Enumeratorclass - Updated tests to expect the new array-based enumeration semantics
Reviewed Changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| src/libraries/System.Collections.Concurrent/tests/ConcurrentBagTests.cs | Adjusted test overrides to reflect array enumerator behavior |
| src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentBag.cs | Changed GetEnumerator to use array enumerator and removed the custom class |
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Tagging subscribers to this area: @dotnet/area-system-collections |
eiriktsarpalis
approved these changes
Jul 7, 2025
This was referenced Jul 7, 2025
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ConcurrentBag's snapshot enumeration semantics are obtained by creating an array of the contents and then enumerating that array. This was done with a custom enumerator out of an abundance of caution about maintaining semantics for operations with undefined behavior, e.g. using Current after MoveNext returns false. We've since stopped trying to maintain bug-for-bug compatibility in such undefined areas. As such, this just deletes the custom enumerator and uses Array's. This not only reduces code but has better performance characteristics (though the whole ToArray as part of enumeration already dominates the enumeration costs).