5. BasePage

This is what your BasePage must do (TestWeb/App_Code/BasePage.cs):

1. Exposes the IMessageBox at the MasterPage to the descendents ;)

   /// <summary>
   /// Gets the message box.
   /// </summary>
   /// <value>El Message box.</value>
   public IMessageBox MessageBox
   {
       get
       {
           MasterPage currentMaster = this.Master;
           while (null != currentMaster)
           {
               if (currentMaster is IMessageBox)
               {
                   return (currentMaster as IMessageBox);
               }
               else
               {
                   currentMaster = currentMaster.Master;
               }
           }
           return null;
       }
   }

2. Registers to the IMessageBox events at the MasterPage

   /// <summary>
   /// Raises the <see cref="E:System.Web.UI.Page.InitComplete" /> event after page initialization.
   /// </summary>
   /// <param name="e">An <see cref="T:System.EventArgs" /> that contains the event data.</param>
   protected override void OnInitComplete(EventArgs e)
   {
       if (null != this.MessageBox)
           this.MessageBox.ResponseMessageBox += new MessageBoxEventHandler(OnResponseMessageBox);
       base.OnInitComplete(e);
   }

3. Exposes a function to descendents to manage the captured events

   /// <summary>
   /// Called when [response message box].
   /// </summary>
   /// <param name="sender">The sender.</param>
   /// <param name="e">The <see cref="MessageBoxEventArgs"/> instance containing the event data.</param>
   protected virtual void OnResponseMessageBox(object sender, MessageBoxEventArgs e)
   {
   }