Will I have any issues if I upgrade from .NET 4.0 to 4.5

Question

I have an existing web app using a combination of MVC and WebForms on .NET 4.0. I want to upgrade to 4.5 to take advantage of some new features in EF 5 (which require 4.5). A couple of questions to hopefully prevent breaking the live site:

1. When I install .NET 4.5 to the hosting server, will it supercede 4.0, or will my 4.0 applications continue to run in 4.0 mode until I change them
2. Are there any breaking changes in 4.5 that are likely to throw my 4.0 app offline
3. If things go bad, can I backout of having installed 4.5?

Solution

1. Once 4.5 is installed then everything that used 4.0 before will use 4.5.
2. There are a handful of compatibility issues that exist. Elegant Code describes the issue we ran into at work when upgrading to 4.5
3. You have to uninstall 4.5 and then re install 4.0 in order to back out 4.5.

Comments

[user1703401]

Yes, no, yes. Just try it, you can't get a warranty here.

[just.another.programmer]

I don't need a warranty, just a way to ward off angry customers if it doesn't work...

[user1703401]

Clearly you are asking for a "but somebody at stackoverflow.com said it would work" warranty. Call Microsoft Support instead.

[just.another.programmer]

Customers don't really care if SO or Microsoft said it would work. They care if it works. I'll get a 4.0 reinstall ready to go just in case and cross my fingers. Thanks.

[John Saunders]

How about you test your app on 4.5 first, before the upgrade?

[p.campbell]

Be clear: the behavior described exists here ONLY for RTM versions of 2010. If you have SP1 for VS 2010, then it's the same solution format.

[SAJ14SAJ]

That is news to me--I simply reported my experience. Yes, my version of VS2010 is RTM, but Windows Update does not report any available updates, even after selecting "Check for Updates" in VS2010. But since I have long been using VS2012 I am not going to worry about it any more. :-)

[Garrett Bates]

How can I confirm #1 is the case for us? We have a .NET 4.0 app that runs on a server with 4.5.1 installed too, and this document contradicts your claim. "If the version of the .NET Framework that the app was built on is not present and a configuration file does not specify a version in a <supportedRuntime> element, the app tries to run on the latest version of the .NET Framework that is present on the user's computer."