How To Calculate Display Form Absolute URL

string serverUrl = item.ParentList.ParentWeb.Site.Url;

if(item.ParentList.ParentWeb.Site.ServerRelativeUrl != "/")           

    serverUrl= item.ParentList.ParentWeb.Site.Url.Substring(0, item.ParentList.ParentWeb.Site.Url.IndexOf(item.ParentList.ParentWeb.Site.ServerRelativeUrl));

string absoluteUrl = String.Format("{0}{1}?id={2}", serverUrl, item.ParentList.DefaultDisplayFormUrl, item.ID.ToString());