How to "clone" an object into a subclass object?

I have a class A and a class B that inherits class A and extends it with some more fields.

Having an object a of type A, how can I create an object b of type B that contains all data that object a contained?

I have tried a.MemberwiseClone() but that only gives me another type A object. And I cannot cast A into B since the inheritance relationship only allows the opposite cast.

What is the right way to do this?

Solution

There is no means of doing this automatically built into the language...

One option is to add a constructor to class B that takes a class A as an argument.

Then you could do:

B newB = new B(myA);

The constructor can just copy the relevant data across as needed, in that case.