Portable code between Silverlight, WPF and more

I had some talks about multi-targeting between WPF and Silverlight. The usual approach to achive this was to have one projects for each platform and use file linking to avoid duplicated code. This was working quite well but was a bit of a hassle as well.

With .NET 4.5 Micorsoft gave developers a new tool to solve the problem: portable libraries! There is a new project template called Portable Class Library. If you select it, a dialog box appears asking you which platforms should be targeted:

The selected settings can later be changed using the project properties. The class library can be directly referenced in a Silverlight project as well as in WPF, which was not possible before: an assembly targeted to the .NET Framework could not be referenced from Silverlight.

Some drawbacks...

As every nice feature, this has its drawbacks as well. Of course in the PCL project you can only use a subset of all supported plattforms. On MSDN is a list of supported features. A PCL project can not reference platform specific assemblies - it is limited to other PCL assemblies.
A major thing I miss in contrast to the old school multiple DLL with shared files approach is, that you can no longer use compiler directives to use platform specific code. Neither can you effectivly determine on what platform your code is running. On the one hand that is the idea of PCL, on the other hand it seems like a limitiation to me.

And a conclusion...

I think it all comes down to careful plannig and using the right tool for the right case. I think you can not discard the mulit-assembly approach completely if you want to get most out of mulit targetign. But PCL are a handy way to have real common code - the one that does not need to know the platform it is running on - in a common location. As suggested on MSDN, platform specific code can then be implemented in platform specific assemblies that inherit from your PCL base classes. This yields to a mixed mode between the old and the new approach.