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

问题:

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?

回答1:

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.

回答2:

One small gotcha you should be aware of if it matters to you is this:

Visual Studio 2010 and Visual Studio 2012 share a format for solution and project file, which makes it very easy for teams who don't all use the same browser version, or for you to upgrade transparently.

However, as soon as you tie a project to .NET 4.5, Visual Studio 2010 will not be able to open or edit the project.

I learned this the hard way when I switched to .NET 4.5, in order to get access to some of the new HttpListener functionality, and lost access to Visual Studio 2010 and my old Rescharper license.