connectionStrings configSource in App.config not working

If you added the file yourself, the build action (in the file properties) may not have been set correctly.

The Copy to Output Directory option needs to be Copy if newer or Copy Always to ensure that the .config file ends up in the bin directory, otherwise it will not be there and trying to load the configuration will fail.

Right Click on file and then Click properties

Change to "Copy always" or "Copy if newer"

I had the same issue and Oded solution works for me. But I will just precise that to find out how to change the file "Copy to Output Directory option" to be "copy if newer or copy always", you have to

• rigth click on the file
• select properties
• go to advance
• then will see copy to output directory and choise copy if newer or copy always

This helped me, I hope it will help you too