Dictionary Keys.Contains vs. ContainsKey: are they

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 be functionally different?

Also, isn't Keys.Contains almost guaranteed to be slower?

Answer 1

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>.

Answer 2

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).