C#

// In WindowsApplication4
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Runtime.InteropServices;

namespace WindowsApplication4
{
  public class Form1 : System.Windows.Forms.Form
  {
    public Form1()
    {
      DataLayer.Data data = new DataLayer.Data();
      Text = data.GetMySetting();
    }
    
    protected override void Dispose( bool disposing )
    {
      base.Dispose( disposing );
    }

    [STAThread]
    static void Main() 
    {
      Application.Run(new Form1());
    }
}

// In ClasLibrary1
using System;

namespace DataLayer
{
  public class Data
  {
    public Data() {}

    public string GetMySetting()
    {
      return System.Configuration.ConfigurationSettings.AppSettings[ "MySetting" ];
    }
  }
}

<configuration>
   <appSettings>      
      <add key="MySetting" value="I love C#" />
   </appSettings>
</configuration>

Client Application <-> Business Layer <- INTERNET -> Web Service <-> Data Layer
Windows Forms App      Class Library                 Web Service     Class Library

Client Application

uses the <applicationName>.exe.config file for the client application

Business Layer

uses the <applicationName>.exe.config file for the client application

Web service

uses the web.config file for the Web service

Data Layer

uses the web.config file for the Web service

object o = MyFormatter.Deserialize(MyStream);
Console.WriteLine(o.GetType().Fullname);