Is there any difference between targeting an application to Framework 4 or Framework 4.5 if you don't use any 4.5 APIs?

Question

after looking at these posts:

http://www.hanselman.com/blog/NETVersioningAndMultiTargetingNET45IsAnInplaceUpgradeToNET40.aspx

http://weblog.west-wind.com/posts/2012/Mar/13/NET-45-is-an-inplace-replacement-for-NET-40

and asked this question:

Is there any difference between building / running an application targeting .Net 4 having installed .Net 4 or .Net 4.5?

Now I have a similar but different question. As the CLR is replaced by .Net 4.5 is there any difference if I change a working application from targeting .Net 4 to target .Net 4.5? Only this change.

Do I need to regression the application or can I be 100% that the application will work the same as the CLR is the same and I am not using any new feature of .Net 4.5?

Answer 1

As mentioned by Hans Passant in the comments,

• a .NET 4.0 application will run on Windows XP where a .NET 4.5 application doesn't

But there's a bit more than that technically

• .NET 4.5 applications will not run on .NET 4.0 framework, so you also loose Windows Vista and Vista SP1 as well as Server 2003 and Server 2008.
• .NET 4.5 is preinstalled on Windows 8 ans Server 2012, so you might need to adapt your installer if it comes with .NET
• Itanium IA64 is supported in .NET 4.0 but not in .NET 4.5

and to do organizationally

• update manuals and the company website
• follow promised phase-out times, e.g. announce the discontinuation of OS some months in advance
• ...