Simplest way to form a union of two lists
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
The easiest way is to use LINQ's Union
method:
var aUb = A.Union(B).ToList();
Using LINQ's Union
Enumerable.Union(ListA,ListB);
or
ListA.Union(ListB);