The new Async Targeting Pack for Visual Studio 11 Beta, just announced by Microsoft, will allow developers to use asynchronous programming features in .NET 4 or Silverlight 5 applications. These features were previously provided in the Async CTP.
Developers seeking to use these features previously had to choose between using Visual Studio 11, or using Visual Studio 2010 with the Async CTP. The Targeting Pack fixes issues with DLL incompatibilities between the Async CTP and Visual Studio 11. It is not supported in Visual Studio 2010.
The async/await keywords and Task-based programming are available in C# and VB. There are some slight variations in the usage of these features in .NET 4 or Silverlight 5 with the Async Targeting Pack versus .NET 4.5. These differences are detailed in the release notes.
As of this release, Microsoft will no longer be updating the Async CTP. The unsupported CTP will remain available for developers working with Windows Phone or Azure applications in Visual Studio 2010, though Microsoft has not specified how long this availability will continue.
The Async Targeting Pack for Visual Studio 11 is available from NuGet under the name Microsoft.CompilerServices.AsyncTargetingPack.