How to get code library targetting .Net 4 to work with .Net 3.5 application?

Why not just build your libraries against . NET 3.5?

Why not just build your libraries against . NET 3.5? If they're trying to use .

NET 4 features then obviously they won't work against your . NET 3.5 webapps anyway If they're not, then using . NET 3.5 should be fine There are some caveats around interop and mixed-mode assemblies, but for pure-C# assemblies it shouldn't be a problem.It's possible to have different build configurations targeting different versions of the framework, but a bit of a pain - you'll need to mess with the project files manually.

See the project files for MiscUtil as an example. I don't see why you were happy with the . NET 2 / .

NET 3.5 situation before - it's not very significantly different to that. Of course, updating your webapps to run on . NET 4 would probably be the best solution, going forward.

It's a DAL and I was hoping to use EF4. – Bill Martin Dec 29 '10 at 15:49 1 @asp316: Then how were you expecting it to work from . NET 3.5?

– Jon Skeet Dec 29 '10 at 15:50 @Skeet - I didn't think it would work. Figured I'd ask if there was an interop trick to allow something like this to work to hold me over until my web apps are upgraded to net4. – Bill Martin Dec 29 '10 at 15:57 @asp316: But you claimed you were fine with .

NET 3.5 and . NET 2 - how would you try to make a . NET 2 app use a .

NET 3.5 API? – Jon Skeet Dec 29 '10 at 17:10 1 @asp316: . NET 2/3/3.5 used the same CLR, but the change to use CLR 4 is unlikely to have very much impact on you.

Upgrading an app to . NET 4 should be no harder than upgrading from 2 to 3.5, for most apps. – Jon Skeet Dec 29 '10 at 20:26.

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