using System;
using System.Linq;
using System.Collections.Generic;
using System.Diagnostics;

public class Test
{
 
	public static void Main()
	{
           var rnd = new Random();
            var List1 = new List<string>(); 
            var List2 = new List<string>(); 
            for (int i = 0; i < 100000; i++)
                List1.Add(rnd.Next(100000, 1000000).ToString());
            for (int i = 0; i < 500000; i++)
                List2.Add(rnd.Next(1000000000, int.MaxValue).ToString());

            var st = new Stopwatch();
            st.Start();
            var match = from str1 in List1
                        join str2 in List2
                        on str1 equals (str2.Length < 6 ? str2 : str2.Substring(0, 6))
                        select str2;

            int matchCount = match.Count();
            st.Stop();
            Console.WriteLine("Elapsed in {0} ms. {1} matches found",st.Elapsed.TotalMilliseconds,matchCount);
	}   
}