Conditional operator doesn't work with two types that inherit the same base type

Question

How come the conditional operator (?:) doesn\'t work when used with two types that inherit from a single base type?

The example I have is:

Answer 1

The conditional part tries to resolve itself regardless of the variable it's being assigned to. The compiler gives a warning that it can't determine which class to use as a return value, because RedirectToRouteResult can't be cast the same as RedirectResult, as far as the conditional part is concerned. However if just one side is cast to the base class, the other is implicitly cast as well, so casting the first would be valid:

 var foo = (someCondition)? 
                  (ActionResult )RedirectToAction("Foo","Bar") :
                  Redirect(someUrl);

but only casting the alternative too:

 var foo = (someCondition)? 
    RedirectToAction("Foo","Bar") :
   (ActionResult)Redirect(someUrl);