You would have to modify the code, and as I mentioned, it can be pretty painful in F# and other ML languages if you already have a bunch of functions that apply to Shape.

Subclassing is a good and useful tool that simply works better in C# than F#. But, just as importantly, class hierarchies are often a poor way to model a problem.

Fair enough, thanks for answers.

You're welcome.

It just occurred to me that you may have been asking about extending Shape in the C# example. In that case, you'd just subclass Shape as normal[1]. Some people argue that sum types shouldn't be extended, but I never really understood that. Basically, C# lets you define unbounded sum types (in the form of subclassing) whereas idiomatic F# more or less demands that you know up front how many variants you're going to need.

[1] Although, sometimes I do define my classes to prevent this by marking the constructor(s) of the abstract base class as private, and then writing the derived class definitions within the body of the base class.

I have seen opinions that C# is good for architecture and F# is good for implementation of components. I might opt for such an approach. Really wish I had a default non-null immutability in C# though.