Find common items across multiple lists in C#

This post will discuss how to find common items across multiple lists in C#.

1. Using `Enumerable.Intersect()` Method

The standard solution to find the set intersection of two sequences is using the Enumerable.Intersect method. It uses the default equality comparer or the specified IEqualityComparer<T> to compare values. It is available in LINQ and can be used as follows:

using System;

using System.Linq;

using System.Collections.Generic;

public class Example

{

public static void Main()

{

var first = new List<int> { 1, 2, 3, 4, 5 };

var second = new List<int> { 2, 4, 6, 8 };

var common = first.Intersect(second);

Console.WriteLine(String.Join(", ", common)); // 2, 4

}

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2. Using `List<T>.FindAll()` Method

The List<T>.FindAll() method retrieves all the elements that match the conditions defined by the specified predicate. It can be used as follows to find the common items across two lists.