An httpModule that allows the master pages in SharePoint to be overridden

MultiMasterHttpModule.cs

using System;
using System.Web;
using System.Web.UI;
using System.IO;

public class MasterPageModule : IHttpModule
{
   public void Init(HttpApplication context)
   {
      context.PreRequestHandlerExecute += new EventHandler(context_PreRequestHandlerExecute);
   }
   void context_PreRequestHandlerExecute(object sender, EventArgs e)
   {
      Page page = HttpContext.Current.CurrentHandler as Page;
      if (page != null)
      {
         page.PreInit += new EventHandler(page_PreInit);
      }
    }
    void page_PreInit(object sender, EventArgs e)
    {
      Page page = sender as Page;
      if (page != null)
      {
         // Is there a master page defined?
         if (page.MasterPageFile != null)
         {
            // only change the application.master files as those are the offenders
            if (page.MasterPageFile.Contains(“application.master”))
            {
               page.MasterPageFile = “/_catalogs/masterpage/MyCustom.master”;
            }
         }
      }
   }

   public void Dispose()
   {
   }
}