Failing to retrieve the values from app.config when called by other application.

I want to get a named setting created in app.config in my class library.  It is working fine as an assembly. But, when this assembly method is invoked by some application, I am getting : Object reference not set to an object for the app setting I am referencing. 

I tried copying app.config in the directory of the application that is invoking my assembly. But still getting the same error.

Any help is highly appreciated.

Thanks,

ex: Case 1: Test1.dll ---> This assembly has a app.config file which has one named key,value.  I have a class method (in the dll) wherein I use this value as a variable value.

2. The above class is referenced in a windows application.

ISSUE: When I run the windows application, it is failing with : Object reference not set..(at the point where the class method is invoked).

Note: When I tried the same app.config within the windows application, it worked well.