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

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