namespace CustomDesign
{
public class CustomPageInit : IHttpModule
{
public void Init(HttpApplication context)
{
context.PreRequestHandlerExecute += new EventHandler(context_PreRequestHandlerExecute);
}

void context_PreRequestHandlerExecute(object sender, EventArgs e)
{
HttpApplication httpApp = sender as HttpApplication;

if (httpApp != null)
{
Page page = httpApp.Context.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.MasterPageFile != null)
{
if (page.MasterPageFile.ToLower().Contains("application.master")) {

SPWeb objWeb = SPContext.Current.Web;

if (objWeb.MasterUrl.Contains("default.custom.master"))
{
objWeb.AllowUnsafeUpdates = true;
objWeb.Update();
string strTmp = page.MasterPageFile.ToLower().Replace("application.master", "application.custom.master");
page.MasterPageFile = strTmp;
}
}
}
}