title: System.MissingMethodException Method not found : NET5 / NET48
body: <p>It might be a dumb question but here is my issue.
  I'm developing a NET5 API that is referencing a NET Standard 2.0 assembly.
  This Net Standard assembly references a NET48 assembly.
  At runtime, I ran into an error message like this:</p>
  <blockquote>
  <p>System.MissingMethodException   HResult=0x80131513   Message=Method
  not found: 'System.String System.String.Concat(System.Object,
  System.Object, System.Object, System.Object, ...)'.</p>
  </blockquote>
  <p>Nothing more in the Stacktrace except the line into the NET48 assembly where the exception occurs.
  Eventually, I started over my API dev in NET48 and the problem is solved.
  It seems to be a compatibility issue but I'm not sure I get the point.
  Could you please explain what's wrong as I thought NET5 API would work with this reference?
  Thanks for your help.</p>