A way of casting a base type to a derived type

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Not soundly, in “managed” languages. This is downcasting, and there is no sane down way to handle it, for exactly the reason you described (subclasses provide more than base classes – where does this “more” come from?). If you really want a similar behaviour for a particular hierarchy, you could use constructors for derived types that will take the base type as a prototype.

One could build something with reflection that handled the simple cases (more specific types that have no addition state). In general, just redesign to avoid the problem.

Edit: Woops, can’t write conversion operators between base/derived types. An oddity of Microsoft trying to “protect you” against yourself. Ah well, at least they’re no where near as bad as Sun.

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