C# find a number common to two or more lists

Question

What is the syntax for finding a list of numbers both present in two or more List? I was looping through when I only needed to check two lists, but now I need to do several... Something like

List<int> commonIds = SELECT id from list1 
                     where list1.contains(id), 
                           list2.contains(id), 
                           list3.contains(id) ... 

Answer 1

Finding the intersection of any number of lists

If you want to find the set of numbers that exist in all of the lists then Enumerable.Intersect is a good way to do so. You don't even have to hardcode the collection of lists, it can be created at runtime:

var lists = new[] { list1, list2, ..., listN }; // dynamically specified

var common = lists.First().AsEnumerable();
foreach (var list in lists.Skip(1))
{
    common = common.Intersect(list);
}

// and now common has the result, e.g.
var listOfCommonEntries = common.ToList();

Finding the union of intersections between master list and each other one

If you want to find the set which includes all common numbers between list 1 and list 2, union all common numbers between list 1 and list N, then it's somewhat different:

var common = Enumerable.Empty<int>();
foreach (var list in lists.Skip(1))
{
    common = common.Union(lists.First().Intersect(list));
}

Answer 2

You can use Enumerable.Intersect:

List<int> commonIds = list1.Intersect(list2.Intersect(list3)).ToList();

It's pretty efficient since it's using a set.

Answer 3

Here is a function,

it returns the union of the intersect of all 2 set combinations.

public static IEnumerable<T> SharedItems<T>(this IEnumerable<IEnumerable<T>> source)
{
    var pairs =
            from s1 in source
            from s2 in source
            select new { s1 , s2 };

    var intersects = pairs
        .Where(p => p.s1 != p.s2)
        .Select(p => p.s1.Intersect(p.s2));

    return intersects.SelectMany(i => i).Distinct();
}

Answer 4

to customize

void Main()
{
     List<int> one = new List<int>() {1, 3, 4, 6, 7};
     List<int> second = new List<int>() {1, 2, 4, 5};

     foreach(int r in one.Intersect(second))
        Console.WriteLine(r);
}