Intersection of List of List

Question

I have a list of lists which looks like the following

public class FilteredVM
{
    public int ID { get; set; }
    public string Name { get; set; }
    public string Number { get; set; }
}

 List<List<FilteredVM>> groupedExpressionResults = new List<List<FilteredVM>>();

I would like to Intersect the lists within this list based upon the ID's, whats the best way to tackle this?

Answer 1

Here's an optimized extension method:

public static HashSet<T> IntersectAll<T>(this IEnumerable<IEnumerable<T>> series, IEqualityComparer<T> equalityComparer = null)
{
    if (series == null)
        throw new ArgumentNullException("series");

    HashSet<T> set = null;
    foreach (var values in series)
    {
        if (set == null)
            set = new HashSet<T>(values, equalityComparer ?? EqualityComparer<T>.Default);
        else
            set.IntersectWith(values);
    }

    return set ?? new HashSet<T>();
}

Use this with the following comparer:

public class FilteredVMComparer : IEqualityComparer<FilteredVM>
{
    public static readonly FilteredVMComparer Instance = new FilteredVMComparer();

    private FilteredVMComparer()
    {
    }

    public bool Equals(FilteredVM x, FilteredVM y)
    {
        return x.ID == y.ID;
    }

    public int GetHashCode(FilteredVM obj)
    {
        return obj.ID;
    }
}

Like that:

series.IntersectAll(FilteredVMComparer.Instance)

You could just write

series.Aggregate((a, b) => a.Intersect(b, FilteredVMComparer.Instance))

but it 'd be wasteful because it'd have to construct multiple sets.

Answer 2

Intersect will work when the type are dead equals, which in your case won't apply because you haven't implemented the GetHashCode and Equals methods, which is the best and complete way.

Thus, If you only intended to take elements that contains in both lists, than the following solution will suit you right.

Assuming list1 and list2 are type List<FilteredVM> than, The most simple way, will be doing this:

var intersectByIDs = list1.Where(elem => list2.Any(elem2 => elem2.ID == elem.ID));

Answer 3

If you are a fan of one-liner solutions you can use this:

List<FilteredVM> result = groupedExpressionResults.Aggregate((x, y) => x.Where(xi => y.Select(yi => yi.ID).Contains(xi.ID)).ToList());

And if you just want the IDs you can just add .Select(x => x.ID), like this:

var ids = groupedExpressionResults.Aggregate((x, y) => x.Where(xi => y.Select(yi => yi.ID).Contains(xi.ID)).ToList()).Select(x => x.ID);

Working Demo