Simplest way to form a union of two lists

Question

What is the easiest way to compare the elements of two lists say A and B with one another, and add the elements which are present in B to A only if they are not present in A?

To illustrate, Take list A = {1,2,3} list B = {3,4,5}

So after the operation AUB I want list A = {1,2,3,4,5}

Answer 1

The easiest way is to use LINQ's Union method:

var aUb = A.Union(B).ToList(); 

Answer 2

If it is a list, you can also use AddRange method.

var listB = new List<int>{3, 4, 5};   var listA = new List<int>{1, 2, 3, 4, 5};  listA.AddRange(listB); // listA now has elements of listB also. 

If you need new list (and exclude the duplicate), you can use Union

  var listB = new List<int>{3, 4, 5};     var listA = new List<int>{1, 2, 3, 4, 5};   var listFinal = listA.Union(listB); 

If you need new list (and include the duplicate), you can use Concat

  var listB = new List<int>{3, 4, 5};     var listA = new List<int>{1, 2, 3, 4, 5};   var listFinal = listA.Concat(listB); 

If you need common items, you can use Intersect.

var listB = new List<int>{3, 4, 5};   var listA = new List<int>{1, 2, 3, 4};   var listFinal = listA.Intersect(listB); //3,4