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We have a set of assemblies that have been compiled as.Net 3.5 (built using VS2008). I now have VS2010 and I'd like to see what .Net 4.0 has to offer.

Do I have to recompile the 3.5 assemblies into 4.0 before I can include them in my project? My first attempts to reference the old libraries doesn't appear to work.

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up vote 3 down vote accepted

Assemblies based on older versions of .NET can be referenced by a newer version but not the other way around i.e. NET 4.0 assembly can use .NET 3.5, 3.0, 2 etc assemblies, but a .NET 3.5 assembly can't reference a .NET 4.0 assembly.

Note that you can only run a NET 4.0 assembly if it is sinstalled on that machine. Also some behaviour between NET versions has changed - significantly NET1.0 to NET 2 for instance.

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Thanks. I wasn't sure if it was dependant upon the project type as it seemed to work for WCF service projects and MVC3 projects but didn't work for a class library. Maybe it was "Fat Fingers" clicking the wrong button. –  TeamWild Oct 31 '11 at 14:40
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