Async/Await Now Available for Windows Phone, Silverlight 4
Microsoft has announced further async/await keyword support for applications not yet targeting .NET Framework 4.5. An updated version of the Async Targeting Pack now provides asynchronous features to Silverlight 4 and Windows Phone 7.5, as well as Portable Class Libraries targeting any of the supported platforms. (Silverlight 5 and .NET Framework 4.0 were supported in the version released earlier this year.)
Microsoft notes that Windows Phone development is not yet available in Visual Studio 2012, but that the ability to target Windows Phone 7.5 will be a part of the upcoming Windows Phone 8 SDK. Visual Studio Web Site projects do not support async/await, but developers can build class libraries containing asynchronous features, and then consume them from web projects.
The new functionality requires Visual Studio 2012 and the NuGet package Microsoft.Bcl.Async. This is beta software, and the BCL team has provided a list of known issues in the Async Targeting Pack.
Developers working with Visual Studio 2010 can take advantage of async/await by using the Visual Studio Async CTP.
Olav Maassen, Liz Keogh & Chris Matts Mar 08, 2014