16
You can use the Intersect method.
var result = one.Intersect(second);
Example:
void Main()
{
List<int> one = new List<int>() {1, 3, 4, 6, 7};
List<int> second = new List<int>() {1, 2, 4, 5};
foreach(int r in one.Intersect(second))
Console.WriteLine(r);
}
Output:
1
4