why connection string is not found from my class library to another class library?

Question

I am having the architecture like this :

Where MVC layer is the presentation layer. EF is class library and Repository is another class library.I am trying to insert data to database from repository by creating the EF context object. Added EF reference into Repository class library. EF having the edmx file. its app.config having the connection string generated by EF. code is :

public bool CreateUser(User _user)
   {
       context.Users.Add(_user);
       context.SaveChanges();
       return true;
   }   

but while executing this I am getting following exception :

No connection string named 'MyEntitiesConnection' could be found in the application config file.

I tried to add same connection string with same name in repository app.config. but not working. anyone have solution ?

Edited: connection string is :

<add name="MyEntitiesConnection" connectionString="metadata=res://*/EF.Entities.csdl|res://*/EF.Entities.ssdl|res://*/EF.Entities.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=Servername\MSSQL2008R2;initial catalog=MyDBName;persist security info=True;user id=sa;MultipleActiveResultSets=True;App=EntityFramework;" providerName="System.Data.EntityClient" />

app.config:

<configuration>
  <configSections>
    <!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
  </configSections>
    <connectionStrings>
        <add name="MyEntitiesConnection" connectionString="metadata=res://*/EF.Entities.csdl|res://*/EF.Entities.ssdl|res://*/EF.Entities.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=Servername\MSSQL2008R2;initial catalog=MyDBName;persist security info=True;user id=sa;MultipleActiveResultSets=True;App=EntityFramework;" providerName="System.Data.EntityClient" />
    </connectionStrings>    
  <entityFramework>
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.SqlConnectionFactory, EntityFramework" />
  </entityFramework>
</configuration>

Answer 1

In any .NET application, only one config file is the natural starting point for looking for configuration information. For web applications, that's the web.config file at the root of the application¹.

Whilst you may have a file called app.config in your repository project (and, indeed, some VS tooling may have added one) or your EF project, it's not used when you try to read configuration information.

The connection string section needs to exist in the web.config of your MVC app.

---

¹For non-web applications, it's the app.config for the project that produces the .exe file and that gets automatically copied as XXX.exe.config during the build.