Using an external .config file in configSource produces error

Question

I was playing around with how to use the Configuration Manager to read/write custom sections in the App.config file for a WPF application in C#. I read this excellent articl

Answer 1

My problem is that I had a was adding a configSource AND a key in the same tag.

Incorrect:

If you delete the "add" tag or move it into your configSource file, the error goes away.

Correct: