Getting "type or namespace name could not be found" but everything seems ok?

This can be the result of a .Net framework version incompatibility between two projects.

It can happen in two ways:

1. a client profile project referencing a full framework project; or
2. an older framework version targeting a newer framework version

For example it will happen when an application is set to target the .Net 4 Client Profile framework, and the project it references targets the full .Net 4 framework.

So to make that clearer:

• Project A targets the Client Profile framework
• Project A references Project B
• Project B targets the full framework

The solution in this case is to either upgrade the framework target of the application (Project A), or downgrade the target of referenced assembly (Project B). It is okay for a full framework app to reference/consume a client profile framework assembly, but not the other way round (client profile cannot reference full framework targeted assembly).

Note that you can also get this error when you create a new project in VS2012 or VS2013 (which uses .Net 4.5 as the default framework) and:

• the referencing project(s) use .Net 4.0 (this is common when you have migrated from VS2010 to VS2012 or VS2013 and you then add a new project)

• the referenced projects use a greater version i.e. 4.5.1 or 4.5.3 (you've re-targeted your existing projects to the latest version, but VS still creates new projects targeting v4.5, and you then reference those older projects from the new project)

Reinstalling nuget packages did the trick for me. After I changed .NET Framework versions to be in sync for all projects, some of the nuget packages (especially Entity Framework) were still installed for previous versions. This command in Packages Manager Console reinstalls packages for the whole solution:

Update-Package –reinstall

I've no idea why this worked, but I removed the project reference that VS2015 was telling me it couldn't find, and added it again. Solved the problem. I'd tried both cleaning, building and restarting VS to no avail.