gist:3916414

## gistfile1.cs
using System;
using System.Diagnostics;
using System.Collections.Generic;
using System.Linq;

namespace test
{
	class Person
	{
		public int Number { get; set; }
		public string Name { get; set; }
	}

	class PersonComparer : IEqualityComparer<Person>
	{
		public bool Equals(Person p1, Person p2)
		{
			return p1.Number == p2.Number;
		}

		public int GetHashCode(Person p)
		{
			return p.Number;
		}
	}

	class MainClass
	{
		public static void Main(string[] args)
		{
			var nameParts = new[] { "fe", "se", "ra", "ko", "il", "as", "os", "de", "ik" };
			var rnd = new Random();

			var list1 = Enumerable.Range(  0, 1000).Select(n => new Person() { Number = n, Name = RandomName(rnd, nameParts) }).ToList();
			var list2 = Enumerable.Range(500, 1000).Select(n => new Person() { Number = n, Name = RandomName(rnd, nameParts) }).ToList();

			var watch = new Stopwatch();

			for (int _ = 0; _ < 5; _++)
			{
				Console.Write("Lists and LINQ merge: ");
				watch.Restart();
				for (int i = 0; i < 100; i++)
				{
					var merged = new List<Person>(list1);
					merged.AddRange(list2.Where(p2 => list1.All(p1 => p1.Number != p2.Number)));
				}
				Console.WriteLine(watch.ElapsedMilliseconds + "ms");

				Console.Write("Dictionary merge: ");
				watch.Restart();
				for (int i = 0; i < 100; i++)
				{
					var dict = list2.ToDictionary(p => p.Number);
					foreach (var person in list1)
					{
						dict[person.Number] = person;
					}
					var merged = dict.Values.ToList();
				}
				Console.WriteLine(watch.ElapsedMilliseconds + "ms");

				Console.Write("LINQ Union and IEqualityComparer: ");
				watch.Restart();
				for (int i = 0; i < 100; i++)
				{
					var merged = list1.Union(list2, new PersonComparer()).ToList();
				}
				Console.WriteLine(watch.ElapsedMilliseconds + "ms");

				Console.Write("HashSet and IEqualityComparer: ");
				watch.Restart();
				for (int i = 0; i < 100; i++)
				{
					var hs = new HashSet<Person>(list1, new PersonComparer());
					hs.UnionWith(list2);
					var merged = hs.ToList();
				}
				Console.WriteLine(watch.ElapsedMilliseconds + "ms\n");
			}

			Console.ReadKey();
		}

		public static string RandomName(Random rnd, string[] nameParts)
		{
			return String.Join("", Enumerable.Range(0, 4).Select(_ => nameParts[rnd.Next(nameParts.Length)]).ToArray());
		}

	}
}