IEnumerable yield and recursion - doesn't work

Question

I'm trying to do something that combines using yield and recursion and I don't really understand why it's not working.

Since my code is complicated, I've generated a simple example with the Fibonacci series:

        private IEnumerable<int> fib()
        {
            int a1 = 1, a2 = 1;
            return fibRec(a1, a2);
        }

        private IEnumerable<int> fibRec(int a, int b)
        {
            int tmp = a;
            a = b;
            b = tmp + b;
            yield return a;
            fibRec(a, b);
        }

Now, I know there are a 1000 ways of solving Fibonacci, but it's not the deal here. this is only an example.

Here is my call:

var nums = fib().Take(50).ToList();

The recursive call doesn't "work".
I get the first yield to work OK and I don't really under stand why this suddenly stops when I try to call myself again.
Doing this with "while(true)" and no recursive call will work just fine.

Answer 1

You need to iterate though the enumerable returned by the recursive call and yield return each of the items explicitly.

    private IEnumerable<int> fibRec(int a, int b)
    {
        int tmp = a;
        a = b;
        b = tmp + b;
        yield return a;
        foreach(int val in fibRec(a, b))
        {
             yield return val;
        }
    }