GetOrCreate(key, defaultValue) won't compile for string, interface or other non-new()-able value types (CS0310)

[matt-edmondson]

What's wrong

The explicit-default overloads of GetOrCreate carry a new() constraint they never use:

• Extensions/DictionaryExtensions.cs ~L31 — GetOrCreate<TKey, TVal>(this IDictionary<TKey, TVal>, TKey key, TVal defaultValue) where TVal : notnull, new()
• Extensions/DictionaryExtensions.cs ~L101 — GetOrCreate<TKey, TVal>(this ConcurrentDictionary<TKey, TVal>, TKey key, TVal defaultValue) where TVal : new()

Both insert the caller-supplied defaultValue and never call new TVal(). The constraint looks copied from the one-argument GetOrCreate(key) overloads, which do need it.

Failure scenario

Each of these fails to compile with CS0310 ("'string' must be a non-abstract type with a public parameterless constructor…"), verified in a probe project against the net9.0 build:

new Dictionary<int, string>().GetOrCreate(1, "one");
new ConcurrentDictionary<int, string>().GetOrCreate(1, "one");
new Dictionary<int, IList<int>>().GetOrCreate(1, new List<int>());

So the overload whose whole point is "I'll supply the value" is unusable for strings, interfaces, abstract types, and records/classes without a parameterless constructor — the cases where the one-argument form can't help either.

Suggested fix

Remove new() from both three-argument overloads (keep notnull on TKey/TVal and the existing runtime null checks). Leave new() on the one-argument overloads. Relaxing a generic constraint is source-compatible for existing callers.

Acceptance criteria

• The three calls above compile and behave as expected (insert when missing, return existing value otherwise).
• Tests added for string and an interface-typed value on both IDictionary and ConcurrentDictionary.

[matt-edmondson]

Triage

• Category: Bug (API constraint)
• Priority: Low. It fails at compile time, so it can't cause a runtime failure, and callers can use TryAdd or indexer patterns meanwhile. The fix is removing an unused new() constraint, which is source-compatible.
• Area / suggested assignee: the two-argument GetOrCreate(key, defaultValue) overloads for IDictionary and ConcurrentDictionary in Extensions/DictionaryExtensions.cs. Owner: @matt-edmondson
• Duplicates / in progress: not a duplicate, and no open PR covers it.
• Next step: drop new() from both overloads that take a default value, keeping notnull and the runtime null checks. Add tests for string and interface-typed values on both dictionary types. This is a good first issue.

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