Problem
You’ve added the necessary using System.Configuration to your class yet the error
persists:
The type or namespace name 'ConfigurationManager' does not
exist blah blah in the namespace 'System.Configuration' (are you missing an
assembly reference?)
So are you missing an assembly reference?
Yes. If you get the error, you are referencing the incorrect assembly.
Solved
Load the correct version of the
Assembly for your .NET Framework
It’s that simple.
