Response to: I have a web application which is using a dll of a class library . I am unable to access the app.config value in the class library How to access app.config ...

Revision 2
You must put the AppSettings value inside the web.config of the application, because it override the app.config from the class library.

Add a reference to System.Configuration namespace in the class library project, them get the value you're expecting from the web.config file, not from the app.config file.