Source code location: ./master_pages/masterpagebase.cs

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

namespace MyProject.MasterPages
{
    public class MasterPageBase : MasterPage
    {
        protected string BaseURL
        {
            get
            {
                try
                {
                    return string.Format("http://{0}{1}",
                                         HttpContext.Current.Request.ServerVariables["HTTP_HOST"],
                                         (VirtualFolder.Equals("/")) ? string.Empty : VirtualFolder);
                }
                catch
                {
                    // This is for design time
                    return null;
                }
            }
        }

        /// <summary>
        /// Returns the name of the virtual folder where our project lives
        /// </summary>
        private static string VirtualFolder
        {
            get { return HttpContext.Current.Request.ApplicationPath; }
        }
    }
}