Make the ConcurrentDictionary GetOrCreate overload atomic - #131
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matt-edmondson merged 1 commit intoSep 26, 2026
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The ConcurrentDictionary overload checked with TryGetValue and then called TryAdd. When two callers missed at the same time, the loser's TryAdd returned false, which fired Debug.Assert in Debug builds. In Release it returned the loser's own defaultValue, which was never stored, so writes to it were silently lost. It now uses GetOrAdd, which returns the stored value to every caller. Fixes #130 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01UHW69XTeuuLbfVdQdJ8uQh
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matt-edmondson
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September 26, 2026 09:52
This was referenced Sep 26, 2026
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Fixes #130
Problem
DictionaryExtensions.GetOrCreate(ConcurrentDictionary<TKey,TVal>, key, defaultValue)did a non-atomic check-then-act:TryGetValue, thenTryAdd. When two callers both missed, the loser'sTryAddreturnedfalse:Debug.Assert(result)fired, which aborts the process.defaultValue, which was never stored, so anything written to it was lost.Fix
The body is now
dictionary.GetOrAdd(key, defaultValue), so every caller gets the value actually stored. TheDebug.Assertand the now-unusedSystem.Diagnosticsimport are removed. The argument validation is unchanged. I did not add the optional factory overload the issue mentions, to keep this PR to the bug.Tests
GetOrCreateConcurrentDictionaryShouldReturnStoredValueWhenAnotherCallerAddsFirstis deterministic. A comparer inserts a rival value on the second hash of the key, which is the moment between the old code's missed lookup and itsTryAdd. The test asserts the returned instance is the stored one.GetOrCreateConcurrentDictionaryShouldReturnSameInstanceToParallelCallersis the stress test from the issue. 1000Parallel.Forcallers add to aConcurrentBag, and the stored bag ends up with 1000 items.With the fix reverted:
Assert.AreSame.Debug.Assertaborts the test host (exit 134).With the fix, the full suite passes in Debug and Release (136/136).
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https://claude.ai/code/session_01UHW69XTeuuLbfVdQdJ8uQh
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