Question

Starting with a basic class:

public class Car
{
  public string Name {get;set;}
}

I can then create a list of these cars

List<Car> cars = new List<Car>();

The new step is to have a List of this List, like this:

List<List<Car>> allListsOfCars = new List<List<Car>>();

After populating allListsOfCars, I want to pass it to a function which will return me the cars which exist in every List list.

I know it sounds confusing, so I’ll try explain a bit more.

If I have ListA, ListB, ListC all of type List – and now combine these into 1 holding list(The list of a list), then how can I get back all the cars that exist in every list? For example if a car only exists in ListA, then I’m not interested, it needs to exist in ListA AND ListB AND ListC, then I want it added to the result set and returned.

Thanks in advance.

Answer 1

You need to find the composite intersection of all the sublists.

IEnumerable<Car> result=allListsOfCars.FirstOrDefault();
if(result!=null)
{
    foreach(var sublist in allListsOfCars.Skip(1))
    {
        result=result.Intersect(sublist);
    }
    //enumerate result to run the query
}

It might be possible to rewrite using the Aggregate operator to eliminate the loop, but Aggregate never reads very well IMO.

If the lists are long, you’ll probably get a decent speed increase using a HashSet

IEnumerable<Car> fst=allListsOfCars.FirstOrDefault();
if(result!=null)
{
    HashSet<Car> hs=new HashSet<Car>(fst);
    foreach(var sublist in allListsOfCars.Skip(1))
    {
        hs.IntersectWith(sublist); //in-place operation
    }
    //enumerate hs
}

Make sure that you Car class implements equality members and GetHashCode correctly, otherwise neither of these approaches will work as expected.