ConcurrentDictionary.GetOrCreate(key) throws ArgumentException when two threads race on a missing key #134

What's wrong

PR #131 (commit 01b92d0) made the ConcurrentDictionary<TKey,TVal>.GetOrCreate(key, defaultValue) overload atomic by delegating to GetOrAdd (Extensions/DictionaryExtensions.cs:65-99). The one-argument form has no ConcurrentDictionary overload, though. concurrentDict.GetOrCreate(key) binds to the IDictionary overload at Extensions/DictionaryExtensions.cs:20:

public static TVal GetOrCreate<TKey, TVal>(this IDictionary<TKey, TVal> dictionary, TKey key) ... => GetOrCreate(dictionary, key, new TVal());

Inside that method dictionary has the static type IDictionary, so the call goes to the non-atomic IDictionary 3-argument overload (:31-58):

if (dictionary.TryGetValue(key, out TVal? val)) { return val; }
dictionary.Add(key, defaultValue);   // IDictionary.Add on ConcurrentDictionary throws if the key now exists

Failure scenario

1. Thread A calls d.GetOrCreate(1) on an empty ConcurrentDictionary<int, List<int>>, and TryGetValue misses.
2. Thread B does the same, and its TryGetValue also misses.
3. Thread A runs Add(1, listA) and succeeds.
4. Thread B runs Add(1, listB), which throws ArgumentException ("An item with the same key has already been added").

• Expected: both callers get the stored instance, as the (key, defaultValue) overload now guarantees.
• Actual: the losing caller crashes.

How verified

Built the library for net10.0 and ran a console program. In each of 2000 iterations, 8 tasks released through a Barrier all call d.GetOrCreate(1) on a fresh ConcurrentDictionary<int, List<int>>. Output:

GetOrCreate(key) on ConcurrentDictionary: 1466 ArgumentExceptions, 0 iterations with differing instances

The existing test GetOrCreateConcurrentDictionaryShouldReturnSameInstanceToParallelCallers only covers the two-argument form, so it does not catch this.

Suggested fix / acceptance criteria

• Add a ConcurrentDictionary overload for the one-argument form, for example public static TVal GetOrCreate<TKey,TVal>(this ConcurrentDictionary<TKey,TVal> dictionary, TKey key) where TKey : notnull where TVal : new() => dictionary.GetOrAdd(key, _ => new TVal());, with the same null checks. Alternatively, have the IDictionary path detect ConcurrentDictionary and call GetOrAdd.
• Add a test that runs Parallel.For over dictionary.GetOrCreate("key1").Add(i) on a ConcurrentDictionary<string, List<int>>. It should not throw and should end with 1000 items in one list.