Polymorphism doesn't work on roles: it works on classes.
So if you have a User class, and Admin class, then you can create a "GoHome" method that exhibitys polymorphism:
public void GoHome(User u) { GoPage("userhome.aspx?u=" + u.Name); }
public void GoHome(Admin a) { GoPage("adminhome.aspx?u=" + a.Name); }
But you cannot use polymorphism based on the value of a variable:
public void GoHome(userType==User) { GoPage("userhome.aspx?u=" + u.Name); }
public void GoHome(userType==Admin) { GoPage("adminhome.aspx?u=" + a.Name); }
It won't compile, or work.