using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
class Test
{

    static void Main()
    {
        string[] test = new string[] { "a", "sdj", "bb", "d444"};
        var expensivePrint = false;
        IEnumerable<int> ls = test.Select(s => { if (expensivePrint) { Console.WriteLine("Doing expensive math"); } return s.Length; });
        expensivePrint = true;
        foreach (var v in ls)
        {
            Console.WriteLine(v);
        }
        Console.WriteLine("If you dont think it does it everytime, lets try it again");
        foreach (var v in ls)
        {
            Console.WriteLine(v);
        }
    }
}