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

c# performance dictionary equivalence
user420667 picture user420667 · Nov 23, 2011 · Viewed 17k times · Source

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

SLaks picture SLaks · Nov 23, 2011

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.

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>.

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