[matt-edmondson]

Fixes #148

What was wrong

#141 (for #140) made only the IEnumerable<T> extension defer to a source's own IDeepCloneable.DeepClone(). A type such as class Tags : List<Item>, IDeepCloneable has no instance DeepClone(), so tags.DeepClone() binds to the more specific List<T> overload. That overload cloned element by element into a plain List<T>, and the subclass's type and its Label were lost. The same happened for HashSet, Dictionary, Queue and the other concrete overloads.

The path #141 did add also called cloneable.DeepClone() without opening the clone context. So an item the collection held twice came back as two clones, against #132.

Change

• New private helper CloneItself. It runs first in every collection overload whose type can be subclassed: IEnumerable, IDictionary, Dictionary, SortedDictionary, SortedList, OrderedDictionary (.NET 9+), ConcurrentDictionary, ReadOnlyDictionary, IReadOnlyDictionary, HashSet, List, Queue, LinkedList, SortedSet, Stack, ConcurrentStack, ConcurrentQueue and ConcurrentBag. Arrays and the immutable and frozen types can't be subclassed, so their overloads are unchanged.
• The helper calls the source's own clone inside CloneContext.Enter(), so one item held twice clones to one shared clone.
• Re-entrancy guard (beyond the issue): an implementation may build its clone by calling the base overload on itself, e.g. clone.AddRange(DeepCloneContainerExtensions.DeepClone<Item>(this)). Deferring to IDeepCloneable in every overload would make that recurse until the stack overflows. A [ThreadStatic] reference set of the sources currently running their own clone handles this. Such an inner call falls through to the element-wise clone, which is what it got before.
• CloneContext.ReferenceComparer goes from private to internal so the guard can reuse it.

#149 (registering collections in the clone context) changes the top of the same overloads. The triage suggests doing it in this helper, but it is a separate behaviour change and is left to its own PR. CloneItself is the natural place to add its TryGetClone/Register.

Tests

New in CloneableCollectionTests:

• List_, HashSet_, Dictionary_ and Queue_DeepClone_OfIDeepCloneableSubclass_ShouldUseItsOwnClone. Each is the issue's Tags repro through that overload. The clone must be the subclass, keep Label == "x" and hold cloned items.
• Enumerable_DeepClone_OfIDeepCloneableListSubclass_ShouldUseItsOwnClone: the same subclass through IEnumerable<T>. This already passed and stays in as a guard.
• Enumerable_DeepClone_OfIDeepCloneableCollectionHoldingOneObjectTwice_ShouldShareOneClone: the issue's Bag holding x twice.
• List_DeepClone_OfSubclassWhoseCloneCallsTheBaseOverload_ShouldNotRecurse: the re-entrancy guard.

Without the library change, 6 of the 7 new tests fail (wrong runtime type, or two clones of the shared item). The guard test fails too, because before the change the type was lost. Full suite on Linux / .NET 10: 168/168 passed. The library builds for net10.0, net9.0, net8.0, netstandard2.1 and netstandard2.0.

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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