namespace TestApp
{
    public static class App
    {
        public static string SiteRootUrl;
    }

    public class TestApplication : HttpApplication
    {
        protected void Application_BeginRequest(object sender, EventArgs e)
        {
            if (string.IsNullOrWhiteSpace(App.SiteRootUrl))
            {
                App.SiteRootUrl = Request.Url.GetLeftPart(UriPartial.Authority) + Request.ApplicationPath;
            }
        }

        protected void Application_End()
        {
            // use App.SiteRootUrl
        }
    }
}

System.Web.Mvc.WebFormViewEngine
System.Web.Mvc.RazorViewEngine