Casting to an unknown type

Hi,
Can anyone tell me how to cast this bit of code back to a Car type?  In the example I know its a type Car but what if I didn't? 
The reason I ask is because I'm writing a little debug method which lists all the Session variables by looping through Session.Contents and therefore I can work out the type, but how use it when casting?  (the line   Car c = Session[sn] as Car; )

Car car = new Car(); car.Model = "Golf"; car.Cc = 1600; Session["Car"] = car; Debug.Print(Session["Car"].GetType().ToString()); object st = Session["Car"].GetType(); String sn = st.ToString(); Debug.Print(sn); Car c = Session[sn] as Car;
...