Mono! Lots of ASP.Net projects (like Nancyfx or SignalR) are actually built on Mono. I think there's some hope that web dev on Windows isn't dead.

I tried Keepass2 with mono on Linux and it's pretty slow and unnatural UX experience. Much like running Wine. On OSX Keepass2 mono just froze constantly and looked weird. I've had similar experience with other cross-platform apps.

Total cross-platform is a pipe dream. Just like Java or more recently HTML5/JS cross-platform mobile apps, they are almost always a half-baked solution compared to proper native OS apps. The UX and performance will never be consistent enough to make it better than just investing in native development (Note I'm talking about software, not simple content consumption apps where HTML5 in browser is fine).

Having primary operating system -> tablet -> phone is a good enough goal. They should focus 100% and make it as stable ass possible.

Naturally mono probably has some quirks, which you need to keep in mind during development. However, you can make successful apps using it. The Unity3D engine uses mono on linux, and the games using it run fine (even on my laptop).

Mono makes no pretext of offering cross-platform GUI live, and so any app that attempts to use the same GUI on multiple platforms using winforms on Linux or gtk on win32 is going to be ugly. They're up front about this.

KeePass is a bad example. It's a Windows Forms app. If they wanted to properly support Linux, they could continue to share most of their C# code, but write a tailored UI using something like GTK#.

Nobody is truly advocating "write once, run anywhere".

How is supporting two desktop operating systems any less realistic than supporting a desktop operating system and at least two separate mobile form factors on three different platforms? Both have the potential to suck if one platform doesn't get enough attention.

I think Qt does it decently, as far as the desktop platforms are concerned.

I mean look at something like Origin or the Battle.net desktop app.

I would also say that wxWidgets makes it pretty easy, particularly as you have the source. I know there is a mass of #defines in it etc. but I have found it very useful and reliable across the 3 major platforms (Win, Linux, Mac). And it renders natively on them, unless you force wxUniversal (which does its own rendering of controls, like Qt does). I would even argue that Qt does a worse job of making stuff look native, particularly on Mac OSX; wxWidgets controls are native on Mac OSX, as underneath it is Obj-C mm files everywhere.

Miguel de Icaza considers Linux dead (http://tirania.org/blog/archive/2013/Mar-05.html). It shows. Last time I tried to compile latest version of mono on ubuntu (a couple of months ago) it was a painful process of missing dependencies and compiler errors. In the end, I couldn't make it run.

in theory Microsoft COULD support linux directly,would be a bit of work, but they would not even need mono.

Now from a business persperctive would it make sense? i dont know,Microsoft business relies heavily on Windows. But hey,anything's possible.

Problem with mono is ,mono or not,.net dev without VS kind of sucks,and VS is Windows bound.