using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var Items = new List<A>();

        var item = new A{ItemID = 25};

        var test = from i in Items

                where i.ItemID == 25

                select i.Values.First(v => v == 69); // no exception

        item.Values.Add(75);

        Items.Add(item); 

        Console.WriteLine(Items.First().ItemID.ToString());

        Console.WriteLine(Items.First().Values.First().ToString());

        test = from i in Items

            where i.ItemID == 25

            select i.Values.First(v => v == 69); // why no exception?

        try{

            var count = test.Count();

        }

        catch{

            Console.WriteLine("exception");

        }

    }

}

​

    public class A

    {

        public A()

        {

            Values = new List<int>();

        }

        public int ItemID { get; set; }

        public List<int> Values {get;set;}

    }