why my pre task step not finish and excute next step?

[sonygod]

private static System.Collections.IEnumerator F1Async(double x, Action completed)
{
for (int i = 0; i < 5; i++)
{
yield return null;
}

        Console.WriteLine("F1Async 終了");
        var result = F1(x);
        completed(result);
    }

I think task will excute next step until I call completed(result);

but current version still excute next step?

how to change to fixed that?