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

After searching the 'net and not really coming up with something I liked (or that worked), I slept on it and came up with this. My SearchResult is similar to your Option. It has an EmployeeId in it and that's the thing I need to be common across lists. I return all records that have an EmployeeId in every list. It's not fancy, but it's simple and easy to understand, just what I like. For small lists (my case) it should perform just fine—and anyone can understand it!

private List GetFinalSearchResults(IEnumerable> lists)
{
    Dictionary oldList = new Dictionary();
    Dictionary newList = new Dictionary();

    oldList = lists.First().ToDictionary(x => x.EmployeeId, x => x);

    foreach (List list in lists.Skip(1))
    {
        foreach (SearchResult emp in list)
        {
            if (oldList.Keys.Contains(emp.EmployeeId))
            {
                newList.Add(emp.EmployeeId, emp);
            }
        }

        oldList = new Dictionary(newList);
        newList.Clear();
    }

    return oldList.Values.ToList();
}