How about ‘mscorelib’ reference?

Sometimes, it is very difficult to understand what happen behind build process inside of complex solutions. You think you did all correct, and then you write simple line of, which is basically doing ‘nothing’ and then:

“You must add a reference to assembly mscorlib, version=4.0.0”

It means you miss everything in framework. The problem is that you probably are targeting multiple frameworks. For example, I have a project with following targets:

    <TargetFrameworks>netstandard1.3;net46</TargetFrameworks>

The error shown above is related to netstandard1.3. To fix this, you cannot simply go to project references and add reference to mscorelib.

Rather, you should add a reference to following package:

Microsoft.NETCore.Portable.Compatibility

That will make your project compliable.