[BobPalmer]

I've built a few cross-platform apps that work both on WP7 and Win8 - as noted before, Portable Class Libraries is a huge help. My web services were fully reused as were all of my base libraries, just had to redo the UI - and between Silverlight and WinRT I was able to copy and paste a lot of my XAML (just a few namespace changes and a couple of minor mods).

I actually did a talk at a user group where I showed a few different apps, then translated them live between the phone, the web, and the tablet.

[DeepDuh]

Thanks for the input. So you can do cross platform if you use these portable class libraries from start. How about about porting win32 code from the .net era? I would guess that most of today's windows applications are running on those libraries, don't they?

[BobPalmer]

I either use a PCL from the start, or I've taken existing code and moved it into the pCL (there are a few tricks to get around due to pieces missing from the various libraries, since they are an overlapping subset).

So if you have existing .net code, I'd try to plug it into the PCL and see which namespaces, etc. are missing then work backwards from there - even if you can only get some of your code, it's a better start than copy/pasting everything IMO.