Question

Using LINQ I can find matching elements between two collections like this:

        var alpha = new List<int>() { 1, 2, 3, 4, 5 };
        var beta = new List<int>() { 1, 3, 5 };

        return (from a in alpha
                join b in beta on a equals b
                select a);

I can increased this to three collections, like so:

        var alpha = new List<int>() { 1, 2, 3, 4, 5 };
        var beta = new List<int>() { 1, 3, 5 };
        var gamma = new List<int>() { 3 };

        return (from a in alpha
                join b in beta on a equals b
                join g in gamma on a equals g
                select a);

But how can I construct a LINQ query that will return the matches between N number of collections?

I’m thinking if each collection was added to a parent collection, then the parent collection was iterated through using a recursive loop, it may work?

Answer 1

There’s no need to recurse – you can just iterate. However, you may find it best to create a set and intersect that each time:

List<List<int>> collections = ...;

HashSet<int> values = new HashSet<int>(collections[0]);
foreach (var collection in collections.Skip(1)) // Already done the first
{
    values.IntersectWith(collection);
}

(Like BrokenGlass, I’m assuming you’ve got distint values, and that you really just want to find the values which are in all the collections.)

If you prefer the immutable and lazy approach, you could use:

List<List<int>> collections = ...;

IEnumerable<int> values = collections[0];
foreach (var collection in collections.Skip(1)) // Already done the first
{
    values = values.Intersect(collection);
}