DeepCloneFrom empties the destination when dest and source are the same collection instance #64

What's wrong

In DeepClone/DeepCloneContainerExtensions.cs, both DeepCloneFrom<T>(this ICollection<T> dest, IEnumerable<T> source) and DeepCloneFrom<TKey,TValue>(this IDictionary<TKey,TValue> dest, IDictionary<TKey,TValue> source) do:

dest.Clear();
foreach (var item in source)
{
    dest.Add(...);
}

If dest and source are the same collection instance, Clear() wipes it, and the foreach then enumerates the now-empty source — no InvalidOperationException is thrown, because the enumerator is obtained after the clear. The collection ends up permanently empty.

Why it matters / concrete failure scenario

The method exists specifically for in-place cloning — detaching a collection's elements from a shared object graph without swapping out the collection instance itself, e.g. widget.Parts.DeepCloneFrom(widget.Parts) to deep-clone a list's own contents in place. That exact natural usage — passing a collection's own reference as both dest and source — is the one input that silently destroys all the data instead of cloning it.

Suggested fix

Snapshot source before clearing dest, e.g.:

T[] items = source.ToArray();
dest.Clear();
foreach (var item in items) { dest.Add(...); }

(or the dictionary equivalent), so the clear can't invalidate the enumeration source. Alternatively, explicitly guard against ReferenceEquals(dest, source).

Acceptance criteria

• A test calling collection.DeepCloneFrom(collection) (same reference for both arguments) results in collection containing clones of its original elements, not an empty collection.