It's tricky. Even if you write for Windows, you'll be using APIs that only work on there (eg if you write in C++ using MFC or COM, or write using .NET and eg.

It's tricky. Even if you write for Windows, you'll be using APIs that only work on there (eg if you write in C++ using MFC or COM, or write using .NET and eg. find that you can only do certain things with Invoke by calling into system DLLs, which you won't be able to do on other platforms, despite the open source nature of .NET these days). If you write for iPad, you are restricted to their iOS APIs. If you write for Mac OS, you'll be restricted to there, as you use Cocoa. If you write for Android, you'll be restricted to there, as you write using Android's APIs despite the existence of Java and some of its runtime. You could attempt C++ on here but the NDK is a bolt-on. The only true solution seems to be to write in a language that is available across all platforms (C++?) using a library that works across all of them (doesn't exist), but this is non-existent as the ways the systems behave (windowing, application process cycle) is different on each platform and it would be foolish to believe that they should be the same, or that they behave in the same way. I think we should just accept that each platform has its merits and disadvantages and stop aiming for this dream of easy cross-compatibility with little developer effort. On all platforms we have to buy the hardware, sometimes have to buy the tooling, and then have to submit our applications to the respective markets or distribution channels (even on Linux, we can't just chuck our stuff at repos, particularly if we want to sell it). Ultimately if we write for a platform, there will always be lock-in of some sort (eg why can't I run my EXE on Mac?).