Yield

gistfile1.cs

public object Solve()
    {
        return Fibonnaci().Where(i => i % 2 == 0).Sum();
    }

    private static IEnumerable<int> Fibonnaci()
    {
        int a = 1,
            b = 2;
        yield return a;
        yield return b;

        while (b < 4000000)
        {
            var auxiliar = b;
            b += a;
            a = auxiliar;
            yield return b;
        }
    }