how to connect Entity Framework in wpf

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C#

WPF

C#4.0

Hi
I have a class library project and a WPF Project.
I am trying to write code inside class library to connect to a wpf project using entity framework. I get error "Configuration system failed to initialize"

below is the code in App.Config

<connectionStrings>
  <add name="dbQuickPhonebookContainer"
       connectionString="metadata=.\dBase.dbQuickPhonebook.csdl|.\dBase.dbQuickPhonebook.csdl|.\dBase.dbQuickPhonebook.msl;provider=System.Data.SqlClient; provider connection string="Data Source=localhost;attachdbfilename=|DataDirectory|\dbQuickPhonebook.edmx;Initial Catalog=dbQuickPhonebook;Integrated Security=True;MultipleActiveResultSets=False""
      providerName="System.Data.EntityClient" />
</connectionStrings>

below is the code inside my class

public class BusinessLayer 
    {
       
        string connString;

        public BusinessLayer()
        {
           
connString = ConfigurationManager.ConnectionStrings      ["dbQuickPhonebookContainer"].ConnectionString;
        }


    }

Please help me write code inside the class to get the name of connection string.