.NET Framework version history - .NET 4.5 is an in-place upgrade to .NET 4.0

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The .NET Framework can version in two ways. There are "side by side installs" and there are "in place upgrades." A major version means side-by-side and a minor version means in-place.

Side-by-side means that different versions of .NET can live together on the same machine.

In-place upgrade means that the CLR is the same but new libraries are added as well as bug fixes and performance improvements:

There's been some concern about how .NET 4.5 is an "in-place upgrade" of .NET 4. That means .NET 4.5 is still the v4CLR and adds new libraries as well as improvements to the core CLR itself.

Note that this in-place replacement is very different from the side by side installs of .NET 2.0 and 3.0/3.5 which all ran on the 2.0 version of the CLR. The two 3.x versions were basically library enhancements on top of the core .NET 2.0 runtime. Both versions ran under the .NET 2.0 runtime which wasn’t changed (other than for security patches and bug fixes) for the whole 3.x cycle. The 4.5 update instead completely replaces the .NET 4.0 runtime and leaves the actual version number set at v4.0.30319.

Overview of .NET Framework release history
Generation Version number Release date Notes Development tool Distributed with
1.0 1.0.3705.0 2002-02-13 original version Visual Studio .NET N/A
1.1 1.1.4322.573 2003-04-24 first update Visual Studio .NET 2003 Windows Server 2003
2.0 2.0.50727.42 2005-11-07 rewrite of framework Visual Studio 2005 Windows Server 2003 R2
3.0 3.0.4506.30 2006-11-06 WCF,WPF,WF Expression Blend Windows Vista, Windows Server 2008
3.5 3.5.21022.8 2007-11-19 LINQ Visual Studio 2008 Windows 7, Windows Server 2008 R2
4.0 4.0.30319.1 2010-04-12 parallel extensions Visual Studio 2010 N/A
4.5 4.5.50709.17929 2012-08-15 asynchronous programming model Visual Studio 2012 Windows 8, Windows Server 2012