using System;

using System.Collections.Generic;       

using System.Globalization;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

​

        var thing = new List<int> { 1, 2, 3, 4, 4, 5, 6 };

        Console.WriteLine(thing.FirstOrDefault(x => x == 4));

        try

        {

            Console.WriteLine(thing.SingleOrDefault(x => x == 4));

        }

        catch(Exception ex)

        {

            Console.WriteLine(ex);

        }

    }

​

}

​