I had this issue today myself, and realized that a solution might be possible using Reflection and then instantiating a new BaseClass object and then copying the values from the properties of the SubClass object onto the BaseClass object (in this so called extension method).

I won't post the code that solved this for me, but if you do need the code for the above design, try googling something like 'how do I convert an object from one type to another that have the same properties using reflection".