Convert base class object to derived class objcet

Hi experts, I was trying to use CType to convert a base class object to a derived class objcet, but got an invalid cast exception.

Here is class definition:
-----------------------------------
Public Class Car
    ...
End Class

Public Class SportsCar
  Inherits Car
   ...
End Class
----------------------------------

Here is program code:
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dim myCar as New Car("Ford", "Black")

Dim mySportsar As SportsCar
mySportsar = CType(myCar, SportsCar)

My purpose is to make mySportsar a Black Ford, and use the member functions defined in SportsCar. But I got the following error

System.InvalidCastException: Specified cast is not valid

Thanks for your help!