find common items across multiple lists in C#

Question

I have two generic list :

List<string> TestList1 = new List<string>(); List<string> TestList2 = new List<string>(); TestList1.Add("1"); TestList1.Add("2"); TestList1.Add("3"); TestList2.Add("3"); TestList2.Add("4"); TestList2.Add("5"); 

What is the fastest way to find common items across these lists?

Answer 1

Assuming you use a version of .Net that has LINQ, you can use the Intersect extension method:

var CommonList = TestList1.Intersect(TestList2) 

Answer 2

If you have lists of objects and want to get the common objects for some property then use;

var commons = TestList1.Select(s1 => s1.SomeProperty).ToList().Intersect(TestList2.Select(s2 => s2.SomeProperty).ToList()).ToList(); 

Note: SomeProperty refers to some criteria you want to implement.