[c#] Simplest way to form a union of two lists

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}

Using LINQ's Union

Enumerable.Union(ListA,ListB);

or

ListA.Union(ListB);

If it is two IEnumerable lists you can't use AddRange, but you can use Concat.

IEnumerable<int> first = new List<int>{1,1,2,3,5};
IEnumerable<int> second = new List<int>{8,13,21,34,55};

var allItems = first.Concat(second);
// 1,1,2,3,5,8,13,21,34,55

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

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

I think this is all you really need to do:

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

var listMerged = listA.Union(listB);