.NET 4.5 installation effecting applications that target 4.0, why?

Question

We have a team developing (each his part of) an app on VS2010, all targeting .NET 4.0 Some code worked perfect on one machine while on the other it had the strangest problems.

We checked for ages for the problem until we finally started checking solution that are not code related. We ended up comparing the configuration of the platforms - after a lot of testing, we found that when installing .NET 4.5 on the machines it showed problems on, it all worked fine. We tried is since the dev dude that was delivering the dll, had it installed on his platform.

It was strange to swallow, but it worked...

(BTW, if someone has encountered this also, our eery code behaviour was when using C# transactions.)

How is that possible that just having the .NET 4.5 on your computer effects how apps that target .NET 4.0 work?

Answer 1

Since 4.5 is an in-place replacement for 4.0 I'd say "yes, it could", though I'd also say "no, it shouldn't", especially if you don't do any reflection into undocumented internals (which aren't guaranteed to ever work, after all).

http://msdn.microsoft.com/en-us/library/hh367887%28v=VS.110%29.aspx is worth a look, though more would be needed on the details of your error to be sure if it was really down to those changes.

Answer 2

As Jon said, as the 4.5 is an in-pace replacement for 4.0.

Some more resources for you to reference:

From Hanselman

From Rick