dictionary.DeepClone() fails to compile (CS0121 ambiguous call) for Dictionary, SortedDictionary and ConcurrentDictionary, although the XML doc example shows that call

[matt-edmondson]

What's wrong

DeepCloneContainerExtensions has two dictionary extensions:

• DeepClone(this IDictionary<TKey,TValue>) (line 137)
• DeepClone(this IReadOnlyDictionary<TKey,TValue>) (line 164)

Dictionary<,>, SortedDictionary<,>, ConcurrentDictionary<,> and ReadOnlyDictionary<,> all implement both interfaces. For these types neither overload is better than the other, so the call is ambiguous and won't compile:

var d = new Dictionary<string,int>();
var c = d.DeepClone();
// error CS0121: The call is ambiguous between
//   DeepCloneContainerExtensions.DeepClone<TKey,TValue>(IDictionary<TKey,TValue>) and
//   DeepCloneContainerExtensions.DeepClone<TKey,TValue>(IReadOnlyDictionary<TKey,TValue>)

The XML doc example on the IDictionary overload (around line 133) shows this exact call: var clonedDict = originalDict.DeepClone();. The tests already work around the problem with explicit casts:

• DeepClone.Test/DeepCloneTests.cs:317: "Cast to IDictionary to resolve ambiguity"
• DeepClone.Test/SpecializedCollectionTests.cs:264: "explicit cast to IDictionary to resolve ambiguity"

In practice, anyone deep-cloning the most common dictionary type has to cast first.

Reproduction

Add var c = new Dictionary<string,int>().DeepClone(); to any file in a project that references the library. dotnet build fails with CS0121, naming both overloads.

Suggested fix

Add overloads for the concrete types, in the same style as the existing HashSet<T>, SortedSet<T> and Stack<T> overloads. The compiler prefers them over the interface overloads. They can also return the concrete type and keep the comparer, which fixes #77 for these types at the same time:

public static Dictionary<TKey,TValue> DeepClone<TKey,TValue>(this Dictionary<TKey,TValue> source) where TKey : notnull
{
    Ensure.NotNull(source);
    return source.ToDictionary(p => DeepClone(p.Key), p => DeepClone(p.Value), source.Comparer);
}

Add matching overloads for SortedDictionary<,> (keep Comparer) and ConcurrentDictionary<,>.

Acceptance: new Dictionary<string,int>().DeepClone() compiles and returns an independent Dictionary with the same comparer, and the cast workarounds in the tests can be removed.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. It's a compile-time failure on the most common call, and the XML doc example shows that exact call. There is a workaround (cast to IDictionary), and nothing breaks silently at runtime.
• Area / suggested owner: DeepCloneContainerExtensions, the dictionary overloads (maintainer: matt-edmondson)
• Duplicates / in progress: Mostly in progress in Keep the key comparer when deep cloning a dictionary #79, which adds concrete Dictionary<,> and SortedDictionary<,> overloads. That resolves CS0121 for those two types, and Keep the key comparer when deep cloning a dictionary #79 includes tests that call DeepClone() without a cast. Keep the key comparer when deep cloning a dictionary #79 has no ConcurrentDictionary<,> overload, so that part of this issue stays open. ReadOnlyDictionary<,> also implements both interfaces and has no concrete overload in Keep the key comparer when deep cloning a dictionary #79.
• Notes: Extend Keep the key comparer when deep cloning a dictionary #79 before merging, or follow up after it, with a ConcurrentDictionary<,> overload (and possibly ReadOnlyDictionary<,>). Then remove the cast workarounds at DeepCloneTests.cs:317 and SpecializedCollectionTests.cs:264.

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