Dictionary Keys.Contains vs. ContainsKey: are they functionally equivalent?

Question

I am curious to know if these two are functionally equivalent in all cases.

Is it possible that by changing the dictionary\'s default comparator that these two would

Answer 1

Although they are pretty much equivalent for Dictionary<,>, I find it's much safer to stick with ContainsKey().

The reason is that in the future you may decide to use ConcurrentDictionary<,> (to make your code thread-safe), and in that implementation, ContainsKey is significantly faster (since accessing the Keys property does a whole bunch of locking and creates a new collection).

Answer 2

These two functions do exactly the same thing.

Keys.Contains exists because Keys is an ICollection<TKey>, which defines a Contains method.
The standard Dictionary<TKey, TValue>.KeyCollection implementation (the class, not the interface) defines it as

bool ICollection<TKey>.Contains(TKey item){ 
    return dictionary.ContainsKey(item); 
}

Since it's implemented explicitly, you can't even call it directly.

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You're either seeing the interface, which is what I explained above, or the LINQ Contains() extension method, which will also call the native implementation since it implements ICollection<T>.