Not to detract from the Microsoft team, but: IIRC Rotor was more of a proof of concept, only supporting .Net 1, and later 2.0. It could not be built that easil

Not to detract from the Microsoft team, but: IIRC Rotor was more of a proof of concept, only supporting .Net 1, and later 2.0. It could not be built that easily either, and not bootstrapped. Early C# (1.1 and 2.0) and the CLR specs were posted to ECMA, then it stopped. I think they published some more of it to ECMA much later, and some of the class platform too but it's still incomplete for .Net to be even remotely called an ECMA standard. Silverlight does work on Mac, but on Linux you have to use Moonlight. All in all, the Mono team went much, much farther than Microsoft, but it is impressive for Microsoft to have shown so much effort and so clear intent towards cross-platform support.