How to Find the Intersection of Multiple Lists Using IEnumerable.Intersect()?

Use IEnumerable.Intersect() to find the intersection of multiple lists

The IEnumerable.Intersect() method in C# allows you to find common elements in two sequences. But what if you have multiple lists and want to identify elements that are present in all lists?

Question:

Given a list of lists of integers:

<code class="language-csharp">var list1 = new List<int>() { 1, 2, 3 };
var list2 = new List<int>() { 2, 3, 4 };
var list3 = new List<int>() { 3, 4, 5 };
var listOfLists = new List<List<int>>() { list1, list2, list3 };</code>

How to use IEnumerable.Intersect() to find the intersection of these lists and get the result List() { 3 }?

Solution 1: HashSet Aggregation

<code class="language-csharp">var intersection = listOfLists
    .Skip(1)
    .Aggregate(
        new HashSet<int>(listOfLists.First()),
        (h, e) => { h.IntersectWith(e); return h; }
    );</code>

This solution uses the Aggregate() method to accumulate a HashSet that represents the intersection of lists. The Skip(1) method ensures that the first list in listOfLists is used as the initial value for the HashSet.

Solution 2: HashSet iteration

<code class="language-csharp">var intersection = new HashSet<int>(listOfLists.First());

foreach (var list in listOfLists.Skip(1))
{
    var intersect = new HashSet<int>(intersection);
    intersection.IntersectWith(list);
}</code>

This solution also uses a HashSet, but it iterates over the remaining lists, creating a new HashSet for each intersection.

Performance considerations:

Performance benchmarks show that in most cases the HashSet solution outperforms using a List. The difference in performance between the foreach method and the Aggregate method is negligible.

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