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Home/ Questions/Q 6366697
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Editorial Team
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Editorial Team
Asked: May 25, 20262026-05-25T00:27:16+00:00 2026-05-25T00:27:16+00:00

Dictionary<TKey, TValue>.KeyCollection Class implements the contains through extension method from IEnumerable , which is

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Dictionary<TKey, TValue>.KeyCollection Class implements the contains through extension method from IEnumerable, which is O(n)

why doesn’t it have a native one with O(1)? it’s kind of HashSet isn’t it?

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  1. Editorial Team
    Editorial Team
    2026-05-25T00:27:16+00:00Added an answer on May 25, 2026 at 12:27 am

    It does implement its own Contains. You can’t implement an interface through an extension method. Note that it implements it explicitly, which means you first have to cast it to ICollection to use it.

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