Changing Target Framework in Visual Studio 2010

Question

I have a Solution with a lot of projects and I rewrote one project with other technologies for which i had to change the target framework from .NET 3.5 to .NET 4.0. All my other projects are in .NET 3.5

Now none of my references seem to work anymore. I have 2k "Are you missing a using directive or an assembly reference?"-Errors

I looked at my references in my other projects and there was a little yellow exclamation mark on the one I changed, that looked a bit like this:

I tried to add the Reference again, some Errors were gone, but that little exclamation mark was still there

Am I doing something wrong? How can I get rid of that exclamation mark? Is it bad to have different .Net versions in one Solution? And will it work if i get my References right?

Answer 1

Is it bad to have different .Net versions in one Solution?

It's not usual to have this scenario, but it should work as long as the projects that are still on .NET 3.5 don't depend on the project you upgraded to .NET 4.0. It would make things easier to manage if you upgraded all the projects, but it's not essential.

And will it work if i get my References right?

Yes.

Try removing and readding the references that are showing the error and also clean and rebuild the solution too.