Generate a randomized list of integers.

RandomList.cs

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

public class Program
{
    public static void Main()
    {
        var count = 20000;
        var stopwatch = new Stopwatch();

        Console.WriteLine("FILLLIST1");
        for (var i = 0; i < 10; i++)
        {
            Run(() =>
            {
                var list = new List<int>();
                FillList1(out _, count);
            });
        }

        Console.WriteLine("FILLLIST2");
        for (var i = 0; i < 10; i++)
        {
            Run(() => FillList2(out var list, count));
        }

        Console.WriteLine("FILLLIST3");
        for (var i = 0; i < 10; i++)
        {
            Run(() => FillList3(out var list, count));
        }

        void Run(Action action)
        {
            stopwatch.Reset();
            stopwatch.Start();
            action();
            stopwatch.Stop();
            Console.WriteLine(stopwatch.Elapsed);
        }
    }

    private static void FillList1(out List<int> list, int count)
    {
        list = new List<int>();
        while (list.Count < count)
        {
            var x = Guid.NewGuid().ToString().GetHashCode();
            if (!list.Contains(x))
            {
                list.Add(x);
            }
        }
    }

    private static void FillList2(out IEnumerable<int> list, int count)
    {
        list = Enumerable
            .Range(1, count)
            .OrderBy(x => Guid.NewGuid());
    }

    private static void FillList3(out IEnumerable<int> list, int count)
    {
        var random = new Random();
        list = Enumerable
            .Range(1, count)
            .OrderBy(x => random.Next());
    }
}