.NET Framework and CLR Versions

In this article, I'm going to attempt to explain what a developer needs to know about the relationship between the .NET CLR version and the .NET Framework version.  I am not attempting to explain what the CLR is or is not.  I am also pretending that version 1.0 did not exist, because I never used it.

First, there was .NET Framework version 2.0 which included .NET CLR version 2.0.  You wrote an application that targets .NET 2.0.  You installed .NET Framework 2.0 on your server, and installed your application.

Then Microsoft released .NET Framework version 3.0 but still included .NET CLR version 2.0.  You installed .NET Framework 3.0 on your server, and since your application was built using a version of the framework that included .NET CLR version 2.0, your application will continue to run with version 3.0 of the Framework.

Then Microsoft released .NET Framework version 3.5 but still included .NET CLR version 2.0.  You installed .NET Framework 3.5 on your server, and again since you application was built using a version of the framework that included .NET CLR version 2.0, you application will continue to run with version 3.5 of the framework.

Then everything changed with .NET Framework version 4.0.  Version 4.0 shipped with CLR version 4.0.  Your app won't work with version 4.0 unless you recompile the app with it targeted to .NET 4.0.  However, on your server, .NET 3.5 will remain installed, so you're app will still work using .NET 3.5 as it did before.

Now, .NET Framework version 4.5 (and 4.5.1) is now out.  These ship with CLR version 4.0.  So if you install 4.5 on your server, your apps built using 4.0 will still work using the new framework.