Sum even fibonacci numbers up to a limit
If you dig a little deeper in the math, it is very straightforward to realize that every third Fibonacci number can be computed with the formula:

\$F_{n+6} = 4 F_{n+3} + F_{n}\$

You can reuse your function if you work the recursion backwards a couple of steps, and do something like:

private static IEnumerable&lt;int&gt; GetEvenFibonacciNumbers()
{
    var first = 2;
    var second = 0;
    while (true)
    {
        var nextFibonacciNumber = first + 4*second;
        yield return nextFibonacciNumber;

        first = second;
        second = nextFibonacciNumber;
    }
} 


This will not only make your code ~3x faster, but will render your other functions trivial to write.