LINQとdynamic使ったサンプル。やっぱLINQ使うならdynamicは必須だね。 C#は今後も良い感じに進化していってほしい。

LinqAndDynamic.cs

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

namespace CExperiments
{
	public class Program
	{
		public static void Main ()
		{
			var sample = LinqSample ();

			foreach (var s in sample) {
				Console.WriteLine ("{0} : {1} {2}", s.Key, s.Value, s.Data);
			}
			/*
			 * 1 : a 111
			 * 3 : c 333
			 * 5 : e 555
			 */
		}

		public static IEnumerable<dynamic> LinqSample()
		{
			var data1 = new [] {
				new { Key = 1, Value = "a" },
				new { Key = 2, Value = "b" },
				new { Key = 3, Value = "c" },
				new { Key = 4, Value = "d" },
				new { Key = 5, Value = "e" },
				new { Key = 6, Value = "f" },
			};

			var data2 = new [] {
				new { Key = 1, Data = "111" },
				new { Key = 2, Data = "222" },
				new { Key = 3, Data = "333" },
				new { Key = 4, Data = "444" },
				new { Key = 5, Data = "555" },
				new { Key = 6, Data = "666" },
			};

			return 
				from 
					a in data1
				join 
					b in data2 on
						a.Key equals b.Key
				where 	
					a.Key % 2 == 1
				select 
					new { a.Key, a.Value, b.Data };
		}
	}
}