Is there any way to write a WinRt (Metro) app that will also work on Windows 7 and Vista?

Question

We can’t just leave our customers that are not able to upgrade to windows 8 for a long time in the larch. However there is demand for a “tablet”/”touch” versio

Answer 1

It is unlikely that we'll see Microsoft push the Metro-style application framework back to past releases due to the level of re-architecting that went into Windows 8.

I agree with Zac on this point. It appears Microsoft is definitely pushing both the technology and usability forward with the introduction of Windows 8 (and Windows Runtime).

The Metro UI is a different UI paradigm. If your using the current Win32 controls (which includes WPF controls), your application will look really dated in Metro. The only way to fix that is to re-implement the UI (your View classes in an MVVM design) using Metro controls. However, C# and majority of the .NET APIs are first class citizens in this new environment. The rest of your application should be fine.

As you already have what I assume a rather large application your best solution would be separate your view from your model-viewmodel. Then you can continue to develop both Windows 8 Metro full-screen touch-friendly awesome interfaces and the "classic" window interface (what we have done for the last x number of years). With good separation, design, and an excellent source control solution (i.e. Perforce) you will able to share a lot of the code base.

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In addition to the answers given to your recent question on Windows Runtime Bill Wagner (one of the many C# bloggers that I follow) has posted summary on WinRT and managed languages conference sessions; it is an good read and recommended if you have a few minutes. One of the things that his summary clarified (in the FAQ at the end) was that the future of .NET as the branding for the framework we use is going to be replaced with Windows Runtime.

Another piece from Bill's blog post:

Some of the .NET APIs are changing for WinRT. I don’t have an exhaustive list, and I’m not sure there is one yet. Other APIs are not exposed via WinRT. (They are still available as .net APIs, just not as Metro / WinRT APIs.)