System.Composition and its child namespaces contain types for developing extensible Windows 8.x Store apps with Managed Extensibility Framework (MEF). These namespaces are part of the .NET for Windows 8.x Store apps subset for the Windows 8 operating system.
These namespaces are not part of the core class library distributed with the .NET Framework. To install these namespaces, open your project in Visual Studio 2012, choose Manage NuGet Packages from the Project menu, and search online for the Microsoft.Composition package.
System.Composition provides classes that constitute the core MEF for Windows 8.x Store apps.
System.Composition.Convention provides types that support using MEF with a convention-based configuration model.
System.Composition.Hosting provides MEF types that are useful to developers of host applications.
System.Composition.Hosting.Core provides MEF types used internally by the composition engine.
For more information about .NET for Windows 8.x Store apps and a list of namespaces and types that it contains, see .NET for Windows Store apps overview in the Windows Dev Center.