using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        int testcount = 10000;

​

        IEnumerable<String> items = new List<String>();

​

        var start = DateTime.Now;

        foreach (var n in Enumerable.Range(0, testcount))

        {

            items = items.Concat(new[] { n.ToString() } );

        }

​

        var concatNotRealized = DateTime.Now.Subtract(start);

        items = items.ToList();

        var concatRealized = DateTime.Now.Subtract(start);

        //  reset items and start

        items = new List<String>();

        start = DateTime.Now;

        foreach (var n in Enumerable.Range(0, testcount))

        {

            items = items.Append(n.ToString());

        }

​

        var appendNotRealized = DateTime.Now.Subtract(start);

​

        items = items.ToList();

        var appendRealized = DateTime.Now.Subtract(start);

        Console.WriteLine("Concat not realized: " + concatNotRealized);

        Console.WriteLine("Concat realized:     " + concatRealized);

        Console.WriteLine("Append not realized: " + appendNotRealized);

        Console.WriteLine("Append realized:     " + appendRealized);

    }

}