ToFrozenDictionary() does not adopt the IEqualityComparer of the source dictionary.

[JeffreyM]

Description

I'm reporting this as a bug, though it seems too obvious to have been overlooked. If it's not a bug, I'm curious about the rationale behind ToFrozenDictionary() not adopting the IEqualityComparer from the source Dictionary.

Reproduction Steps

// Note the case insensitive IEqualityComparer<T> - StringComparer.OrdinalIgnoreCase here.
var dictionary = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase) { ["a"] = 1 };
        
// Assertions will pass
var frozenDictionary = dictionary.ToFrozenDictionary( StringComparer.OrdinalIgnoreCase);
Debug.Assert(frozenDictionary.Comparer == dictionary.Comparer);
Debug.Assert(frozenDictionary.TryGetValue("A", out int _)); 

// Assertions will fail
frozenDictionary = dictionary.ToFrozenDictionary( /* No StringComparer */);
Debug.Assert(frozenDictionary.Comparer == dictionary.Comparer);
Debug.Assert(frozenDictionary.TryGetValue("A", out int _)); 

Expected behavior

I expect the following would satisfy the assertion.

var dictionary = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase) { ["a"] = 1 };
var frozenDictionary = dictionary.ToFrozenDictionary();
Debug.Assert(frozenDictionary.Comparer == dictionary.Comparer);
Debug.Assert(frozenDictionary.TryGetValue("A", out int _)); 

Actual behavior

The actual behavior is that the assertions fail for the expected behavior above.

Regression?

No response

Known Workarounds

Explicitly pass the same IEqualityComparer to ToFrozenDictionary(comparer) as you do when instantiating a dictionary it will be called on. This is a very simple workaround. However, there are times where I would not necessarily know the IEqualityComparer of the source dictionary. Serialization/Deserialization comes to mind.

Configuration

No response

Other information

No response

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[huoyaoyuan]

ToXxDictionary series methods don't operate on a dictionary, they operate on collection of key-value pairs. Thus no concept of comparer will be passed implicitly.

[JeffreyM]

Thank you @huoyaoyuan. That also explains why serializing a Dictionary looses the notion of it comparer. Closing this.