Fastest way to find common items across multiple lists in C#

Question

Given the following:

List> optionLists;

what would be a quick way to determine the subset of Option objects that a

Answer 1

You can do this by counting occurrences of all items in all lists - those items whose occurrence count is equal to the number of lists, are common to all lists:

    static List FindCommon(IEnumerable> lists)
    {
        Dictionary map = new Dictionary();
        int listCount = 0; // number of lists

        foreach (IEnumerable list in lists)
        {
            listCount++;
            foreach (T item in list)
            {
                // Item encountered, increment count
                int currCount;
                if (!map.TryGetValue(item, out currCount))
                    currCount = 0;

                currCount++;
                map[item] = currCount;
            }
        }

        List result= new List();
        foreach (KeyValuePair kvp in map)
        {
            // Items whose occurrence count is equal to the number of lists are common to all the lists
            if (kvp.Value == listCount)
                result.Add(kvp.Key);
        }

        return result;
    }