using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var data    = new List<string>{"A", "B", "C", "D", "E", "F", "G", "H", "I"};

var targets = new List<string>{"C", "D", "E", "F"};

​

var found    = data.Intersect(targets).ToList();

var notFound = data.Except   (targets).ToList();

​

Console.WriteLine("Found:     " + string.Join(", ", found));

    }

}