c# exists in list

List<int> nums1 = new List<int> { 2, 4, 6, 8, 10 };
List<int> nums2 = new List<int> { 1, 3, 6, 9, 12};

if (nums1.Any(x => nums2.Any(y => y == x)))
{
    Console.WriteLine("There are equal elements");
}
else
{
    Console.WriteLine("No Match Found!");
}

var items = (from x in parameters
                join y in myStrings on x.Source equals y
                select x)
            .ToList();

using System;
using System.Collections.Generic;
public class Program {

   public static void Main() {
      List < string > list1 = new List < string > () {
         "Lawrence",
         "Adams",
         "Pitt",
         "Tom"
      };

      Console.Write("List...\n");
      foreach(string list in list1) {
         Console.WriteLine(list);
      }

      Console.Write("Finding an item in the list...\n");
      if (list1.Contains("Adams") == true) {
         Console.WriteLine("Item exists!");
      } else {
         Console.WriteLine("Item does not exist!");
      }
   }
}

You could use a nested Any() for this check which is available on any Enumerable:

bool hasMatch = myStrings.Any(x => parameters.Any(y => y.source == x));
Faster performing on larger collections would be to project parameters to source and then use Intersect which internally uses a HashSet<T> so instead of O(n^2) for the first approach (the equivalent of two nested loops) you can do the check in O(n) :

bool hasMatch = parameters.Select(x => x.source)
                          .Intersect(myStrings)
                          .Any();