Mixed (Native and Managed) Assemblies
Mixed assemblies are capable of containing both unmanaged machine instructions and MSIL instructions. This allows them to call and be called by .NET components, while retaining compatibility with components that are entirely unmanaged. Using mixed assemblies, developers can author applications using a mixture of managed and unmanaged functionality. This makes mixed assemblies ideal for migrating existing Visual C++ applications to the .NET Platform.
For example, an existing application consisting entirely of unmanaged functions can be brought to the .NET platform by recompiling just one module with the /clr compiler switch. This module is then able to use .NET features, but remains compatible with the remainder of the application. In this way, an application can be converted to the .NET platform in a gradual, piece-by-piece fashion. It is even possible to decide between managed and unmanaged compilation on a function-by-function basis within the same file (see managed, unmanaged).
Visual C++ supports the generation of three distinct types of managed assemblies: mixed, pure, and verifiable. The latter two are discussed in Pure and Verifiable Code (C++/CLI).
For more information about coding guidelines for mixed assemblies, see the MSDN article "An Overview of Managed/Unmanaged Code Interoperability" at http://msdn.microsoft.com/netframework/default.aspx?pull=/library/en-us/dndotnet/html/manunmancode.asp.