Mono has always been behind what is happening and has lacked the full support of MS.
Understandable, since Mono is not an MS project. In fact, MS had nothing o do with Mono, it was a project started by Ximian.
My observation is that it is seen as a poor cousin by .NET developers.
Then I have to say you didn't observe very well. Mono is not a cousin, it isn't even a MS project, it was created as a compatible clone of the .NET framework for non-Windows platforms (although Mono works on Windows as well). Of course since its a clone Mono isn't 100% compatible, and if you want to develop cross-platform there are a few things you need to pay attention to.
I agree that Microsoft are finally coming around and changing their policies, so hopefully mono will become mainstream with better support.
Mono *is* mainstream and very well supported on Linux and other non-Windows OSes (including Android), plus now that .Net is open source it will incorporate even more of parts of .Net which will make it even easier to write cross-platform applications.
However it is hard to know what the current status is without actually using it as the devil is in the details.
It's actually not that hard:
http://www.mono-project.com/docs/But like with many other complex things you have to spend some time to become familiar with it.
If Linux/Mac support is possible without compromising other variants then great but I'd primarily focus on the platform that roughly 95% of users are going to use.
Thats what got us into this mess... Chicken and egg situation. Many people use Apple products - I am typing this on an Apple laptop as it was by far the best hardware available for the price at the time. Linux is widely used in fields which are closely related to the electronics industry - e.g. education and software development. And of course in embedded systems.
The electronics and T&M market has become entrenched in Windows, which is frustrating for people used to using multiple platforms - the best platform for the job.
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As ntnico said the electronics market is mostly Windows and Linux, but yes, T&M is mostly Windows for good reasons, simply because most T&M engineers want to create test solutions quickly without fuss, and that is unfortunately easier than on Linux which generally needs a lot more babying.
Apple's OS X isn't an alternative simply because it's a desktop/laptop platform that runs of a very limited set of pretty expensive hardware, plus Apple's habit of cutting down on standard ports and the pretty short support cycles of OS and even hardware make it a pretty unattractive alternative for lab use.
Back to the idea of writing an open source waveform editor, you have two options here: you could use the idealistic way and develop for Linux or Mac as primary platform which I'm sure would make a few Linux and Mac users very happy, but then the vast majority of Siglent/Rigol AWG users will still have to rely on the crap Siglent and Rigol software because they run Windows. Or you can go the practical way and develop for the platform that is used by the majority of users (Windows), which means your software will serve a lot more people (plus you'll more likely to get useful feedback). And if you plan accordingly you can make your application cross-platform so Linux and even OS X isn't completely left out either.