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

These two functions do exactly the same thing.

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

bool ICollection.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.