ASP.NET

<appSettings>
    <add key="ValidationSettings:UnobtrusiveValidationMode" value="None">
  </appSettings>

class Program
  {
    
    static void CompareList<T>(List<T> a, List<T> b, ref List<T> diff)
    {
      foreach(T item in a)
      {
        if(!b.Contains(item))
        {
          if(diff.Count == 0)
          {
            diff.Add(item);
          }
          else if(!diff.Contains(item))
          {
            diff.Add(item);
          }
        }
      }
    }

    /// <summary>
    /// Compare Two Lists of primitive types and return sorted list
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="a">The first list to compare</param>
    /// <param name="b">The second list to compare</param>
    /// <param name="diff">the ref param that returns the sorted list of items not in both lists</param>
    static void ListDifference<T>(List<T> a, List<T> b, ref List<T> diff)
    {
      CompareList(a,b,ref diff);
      CompareList(b,a,ref diff);
      diff.Sort();
    }

    static void Main(string[] args)
    {

      List<int> a = new List<int>{1,2,3,4,5,6,7};
      List<int> b = new List<int>{4,6,7,8,9,10};
      List<int> diff = new List<int>();

      List<string> c = new List<string>{"a", "b", "c", "d","e", "f", "t", "U"};
      List<string> d = new List<string>{"b","e","f","g","h"};
      //List<string> d = new List<string>();
      List<string> diff2 = new List<string>();

      ListDifference(a,b, ref diff);
      ListDifference(c, d, ref diff2);

      Console.WriteLine("-- Result of CompareLists for Int");
      foreach(int i in diff)
      {
        Console.WriteLine(i.ToString());
      }
      Console.WriteLine();
      Console.WriteLine("Results for CompareLists for strings");
      foreach(string s in diff2)
      {
        Console.WriteLine(s);
      }

      Console.Read();
    }
  }