Q: Fastest way to cross check 2 Lists with eachother What would be the most effective method to check whether a string value from list1 exists in list2?
Pseudo Code example
StringList List1 = {"a", "b", "c", "d"};
StringList List2 = {"d", "c", "b", "a"};

foreach (string a in List1)
{
    foreach (string b in List2)
    {
        if(a==b)
        {
             WriteLine("match");
        }
    }
} 

This method is far too slow to be the best solution, the platform I am using is BI#, a derivative off C#. Array.Exists does not exist in this current library.

A: You can use Intersect:
var allIntersections = List1.Intersect(List2);

foreach(string match in allIntersections)
   Console.WriteLine(match);

If you just want to know if at least one intersects:
bool anyExist = allIntersections.Any(); 


A: Converting the List into a comma deliminated string, allows to use a standard InString function in this case a function called StringContains;
 foreach (string a in List1)
{
    if(StringContains(a, NewString))
    {
        WriteLine("Match");
    }
} 


A: bool flag= list2.Except(list1).Any();

This will be true if list1 doesn't contains all from list2.
