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

Answer 1

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

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

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