A significant problem is that once .NET 4.5 is installed, a developer can no longer test how an app behaves under .NET 4, even if the app still has its target f

A significant problem is that once .NET 4.5 is installed, a developer can no longer test how an app behaves under .NET 4, even if the app still has its target framework set to .NET 4. There are several major WPF bugs fixed in .NET 4.5; developers will no longer encounter those bugs locally, but can only find them on a dedicated .NET 4 testing machine, or (more likely) reported from a .NET 4 customer in the field. There's a detailed writeup of the problem here: http://social.msdn.microsoft.com/Forums/vstudio/en-US/c05a8c02-de67-47a9-b4ed-fd8b622a7e4a/if-i-target-net-40-but-run-on-a-machine-that-has-net-45-will-net-40-wpf-bugs-still-be-there?forum=wpf http://social.msdn.microsoft.com/Forums/vstudio/en-US/c05a8c... See also this suggestion: http://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/3095632-make-vs2012-not-hide-net-4-0-bugs-when-targeting- http://visualstudio.uservoice.com/forums/121579-visual-studi...