When Is a Version Not a Version?

Despite the seemingly significant changes in version numbers, versions 3.5, 3.0, and 2.0 of the .NET Framework share the same underpinnings. A version of the .NET Framework corresponds to a particular version of the CLR, which is the software engine responsible for loading, verifying, and executing .NET code, along with specific versions of particular class libraries.

Sometimes an update to the .NET Framework includes a new version of the CLR, as happened with versions 1.1 and 2.0. In these cases, the new version of the CLR is installed alongside the previous version, enabling applications written for the older version to continue to run.

But version 3.0, introduced with Windows Vista in Dec. 2006, did not include any core CLR changes. Instead, it simply added class libraries, such as the Windows Presentation Foundation (shown here as “Presentation.Framework”—the name of the assembly that contains it). Similarly, version 3.5 adds new class libraries, such as LINQ, onto the existing 3.0 infrastructure. In each case, the update also includes fixes for bugs in other libraries but which should not require any change to applications that use them.