invoke() begininvoke() operative differently executing an overridable slight around delegate

can anyone tell me since formula behaves proceed does? comments embedded code...

am i blank something unequivocally apparent here?

using system;
namespace consoleapplication3
{
    open category program
    {
        stationary vacant main(string[] args)
        {
            var c = new mychild();
            c.x();
            console.readline();
        }
    }

    open category myparent
    {
        open practical vacant x()
        {
            console.writeline("executing myparent");
        }
    }

    nominee vacant mydelegate();

    open category mychild : myparent
    {
        open overrule vacant x()
        {
            console.writeline("executing mychild");
            mydelegate md = base.x;

            // following twin calls feeling should act same,
            //  nonetheless act differently!    

            // since does invoke() bottom category approaching here...
            md.invoke();

            // ... nonetheless begininvoke() performs recursive within
            //  child category bottom class?
            md.begininvoke(callback, null);
        }

        open vacant callback(iasyncresult iasyncresult)
        {
            return;
        }
    }
}