I'm realizing there's a number of subtle but important new things in the next version of VS that streamline some previously difficult tasks. For example, there's the .NET Framework, but .NET is also in Silverlight, the Windows Phone, the Xbox, etc. If you create a regular Class Library it has a single Target Framework. However, if you are doing a multi-platform application and you want to maximize your code reuse, you can run into trouble as you may not have all libraries available on the smaller platforms.
Thus, Portable Class Libraries were created. You can get Portable Class Libraries via an extension on Visual Studio 2010 or they are built into Visual Studio 11 Beta.
These Portable Class Libraries (PCLs) will generate a managed assembly that can be referenced by Windows Phone 7, Silverlight, the Microsoft .NET Framework and Xbox 360 platforms. This really helps to maximize reuse of code and reduce the number of projects in multi-targeted application solutions.
I talked about multi-targeting a little in the .NET Versioning and Multi-Targeting - .NET 4.5 is an in-place upgrade to .NET 4.0 post. Visual Studio is smart enough to tell you before compilation what APIs are available. The compiler knows and Intellisense knows. The Reference Assemblies down in C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\ tell you a lot.
If you create a new Portable Class Library, right click on it and hit Properties. You can target specific frameworks and the build system and Intellisense will adjust. Some libraries are on some device. For example, XML Serialization isn't on the Xbox 360, but WCF is on the Windows Phone.
Bill Kratochvil has an article in MSDN Magazine that includes the source for a project that targets Windows Phone 7, Silverlight and WPF with shared libraries. David Kean has an article this month in MSDN Mag on how to Create a Continuous Client Using Portable Class Libraries.
Remember that Portable Class Libraries are constrained by design. You're targeting the lowest common denominator to maximize what you can use between projects. The MSDN article on Portable Class Libraries has a table that show what's available on each platform. You can do MVVM work across Windows, Metro style aopps, Silverlight and Windows Phone, for example.
Here's some good advice that David Kean sent me:
One thing [about] using portable [is that it] doesn’t mean you can’t use platform specific features, you just need to spend a little more time thinking about your dependencies. For example, instead of having a low level class that handles your persistence layer using the File APIs (which aren’t usable/available on Phone, Silverlight, etc), have it instead take work on an abstraction such as a stream, which these gets passed that from the platform specific project. Or have it call through an platform adapter (called out in my article), or inject the abstraction using your favorite IoC container (in my case Autofac, just published a portable version)MSDN Help also shows what works with Portable Libraries as well so you're supported when looking at Help, Intellisense, and at Build time.
The BCL Blog mentioned that they are talking to the Mono guys about this as well. It'd be great to get Mono for Android and other frameworks as appropriate in here as well. I was excited to discover that this work was happening, even though it's been over a year in the making.
For folks like my friends at Rowi who have a great Windows Phone 7 application, or MetroTwit with a great WPF app, I wonder how Portable Class Libraries might change the architecture of their applications and enable cleaner builds and reuse scenarios they haven't thought of.
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