How to add a C++ DLL in Windows Phone 8 (C#) Framework

I am trying to add a C++ DLL to Windows Phone 8 framework in Visual Studio Express 2012.

I have tried following ways

1. Import and invoke through PInvoke

   [DllImport("WP8DLL.dll", CallingConvention = CallingConvention.Cdecl)]

   public static extern int functionReturningInteger();

   Result: This way though there were no compile errors, but when I try to access the method of the DLL it throws System.NotSupportedException.

2. Adding reference in the project properties

   Result: I get the message "A reference to a higher version or incompatible assembly cannot be added to the project"