app.config gives error when accessing database in class

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C#

WinForms

Sir

string connectionString =(string)ConfigurationSettings.AppSettings["SanaSoh"];

This line runs correctly on page, but when run in a class it gives me an error. Somebody please give me an idea for app.confing file. I use app.cofing file and give path of my database, but I don't get access to database from within the class.
Please give me an idea how to use database in a class from win.config by using app.config or directly.

Thank you!