.NET Framework 4 Eases Backward Compatibility

The .NET Framework 4, included with Visual Studio 2010, adds in-process side-by-side compatibility with earlier versions of the Framework. This feature allows an application process to simultaneously run part of its code using the .NET Common Language Runtime (CLR) 4 and other parts using older versions of the CLR. The new compatibility enhancement promises to help organizations develop and deploy applications that use .NET 4 while minimizing conflicts with existing code that use a previous version of the Framework.

Applications Can Load Multiple CLRs

The .NET Framework includes the CLR, a software component for loading and running applications, and class libraries, hierarchically organized collections of code that developers can use in their applications to build interfaces, access databases and files, and communicate over the Web. The CLR has not been revised since .NET 2.0 (released in Nov. 2005), and all .NET releases from .NET 2.0 through .NET 3.5 SP1 (the release prior to 4) contain CLR 2.0, so applications can target any of these releases with few, if any, compatibility issues. There were two CLR releases prior to CLR 2.0: CLR 1.0 and CLR 1.1, which were released with .NET 1.0 and .NET 1.1, respectively. The .NET Framework 4 includes CLR 4 (there was no CLR 3).