Example

            List<int> outer = new List<Int32> { 1, 2, 3, 4 };
            List<int> inner = new List<Int32> { 1, 6, 3, 8 };

            Console.WriteLine(outer.Count);
            Console.WriteLine(inner.Count);
            var result = outer.GroupJoin(
                inner, // the inner list i.e. IEnumerable<TInner>
                o => o, // We're selecting elements from the outer list in this case we just select them all
                i => i, // We're selecting elements from the inner list in this case we just select them all
                (o, g) => g.Select(c => new { Item1 = o, Item2 = c}) // This is our result function, it returns a tuple of each element if each element has an equal value
                .DefaultIfEmpty(new { Item1 = o, Item2 = 0 }))  // This speficies a default value for the tuple if the row doesn't match.
                .SelectMany(g => g) // Flatten into one sequence
                .ToList(); // Convert to a list

            Console.WriteLine(result.Count);
            // result should now contain the tuples
            for (int i = 0; i < result.Count; i++)
                Console.WriteLine("Item1: " + result.ElementAt(i).Item1 + " Item2: " + result.ElementAt(i).Item2);
            Console.Read();