C＃LINQ集合的“内部联接”

Question

Basically I have a IEnumerable<IEnumerable<T>> object and I want to get from it IEnumerable<T> of objects that exist in all of the collections.

I'm trying to code that as elegant as possible, and I'm wondering if there is some LINQ trick of doing that, preferably one-liner. If it can help, both IEnumerable s are ISet .

Currently I'm using a code similar to below:

// Some starting data - collection of collections
HashSet<HashSet<ulong>> data = new HashSet<HashSet<ulong>> { new HashSet<ulong> { 1, 2, 3 }, new HashSet<ulong> { 2, 3, 4 }, new HashSet<ulong> { 3, 4, 5 } };

// Flatten collection of collections into a single collection with unique elements
HashSet<ulong> result = new HashSet<ulong>(data.SelectMany(set => set));

// Remove from it all elements that are missing from 1 or more initial collections
result.RemoveWhere(number => data.Any(set => !set.Contains(number)));

// The result is: HashSet<ulong> { 3 }

It works, but I'm doing several things here such as enumerating initial collection twice, creating a half-ready result and iterating + removing elements from it.

I'm pretty sure there should be an option to do that in one-go with LINQ, I'm basically looking for something like:

HashSet<HashSet<ulong>> data = new HashSet<HashSet<ulong>> { new HashSet<ulong> { 1, 2, 3 }, new HashSet<ulong> { 2, 3, 4 }, new HashSet<ulong> { 3, 4, 5 } };elements
HashSet<ulong> result = data.LinqInnerJoinMagic(set => set);

Does something like this exist in C#? Maybe as an extension method? I tried hard to make use of LINQ Join() but I couldn't find out how to adapt it to my example, I'd appreciate a helpful hand, it's very likely that the answer is so simple that I can't really find it.

Thank you in advance!

Answer 1

You can use HashSet<T>.IntersectWith :

HashSet<ulong> result = data.First();
foreach (HashSet<ulong> set in data.Skip(1))
    result.IntersectWith(set);

Modifies the current HashSet<T> object to contain only elements that are present in that object and in the specified collection.

Answer 2

Aggregate combined with intersect will get you the common elements that exist in all sets.

var orderedData = data.ToList();
var commonSet = orderedData.Skip(1).Aggregate((IEnumerable<ulong>)(orderedData.First()), (c, n) => c.Intersect(n));

Answer 3

Try this:

var result = new HashSet<ulong>(
      data.SelectMany(x => x).Where(x => data.All(z => z.Contains(x)))
);

See MSDN