LINQ ToDictionary initial capacity

Question

I regularly use the LINQ extension method ToDictionary, but am wondering about the performance. There is no parameter to define the capacity for the dictionary and with a list of 100k items or more, this could become an issue:

IList<int> list = new List<int> { 1, 2, ... , 1000000 };
IDictionary<int, string> dictionary = list.ToDictionary<int, string>(x => x, x => x.ToString("D7"));

Does the implementation actually take the list.Count and passes it to the constructor for the dictionary? Or is the resizing of the dictionary fast enough, so I don't really have to worry about it?

Answer 1

Does the implementation actually take the list.Count and passes it to the constructor for the dictionary?

No. According to ILSpy, the implementation is basically this:

Dictionary<TKey, TElement> dictionary = new Dictionary<TKey, TElement>(comparer);
foreach (TSource current in source)
{
    dictionary.Add(keySelector(current), elementSelector(current));
}
return dictionary;

If you profile your code and determine that the ToDictionary operation is your bottleneck, its trivial to make your own function based on the above code.