mylifeinaminute.com

To inherit the master page from the current site being browsed in a custom application page (i.e. a page that resides in the “_layouts” directory), you can use the following PreInit method:

private void Page_PreInit(object sender, EventArgs e)
        {
            System.Web.UI.Page page = sender as System.Web.UI.Page;
            if (page == null) return;
            if (page.MasterPageFile == null) return;

            HttpContext context = HttpContext.Current;
            string currUrl = context.Request.Url.ToString();
            string basepath = currUrl.Substring(0, currUrl.IndexOf(context.Request.Url.Host) + context.Request.Url.Host.Length);
            //Use RawUrl, otherwise it will always use the root web.
            currUrl = context.Request.RawUrl;
            if (currUrl.ToLower().Contains("/_layouts/") &&
                page.MasterPageFile.ToLower().EndsWith("application.master"))
            {
                currUrl = currUrl.Substring(0, currUrl.ToLower().IndexOf("/_layouts/"));
                using (SPSite site = new SPSite(basepath + currUrl))
                {
                    using (SPWeb web = site.OpenWeb(currUrl))
                    {
                        page.MasterPageFile = web.MasterUrl;
                    }
                }
            }
        }

Just add the code above to your code behind, and your custom page will now inherit the look and feel of the site it is currently in.