C# : The name 'ConfigurationManager' does not exist in the current context

Error Display as below

Adding the System.Configuration as reference to all the projects will solve this.

1. Go to Project -> Add Reference

2. In the box that appears, click the All assemblies list tab in the left hand list.

3. In the central list, scroll to System.Configuration and make sure the box is checked.

4. Click ok to apply, and you'll now be able to access the ConfigurationManager class.

Sumber : https://stackoverflow.com/a/52928968/10763264