Get All users in Active directory through Visual Webpart - Sharepoint 2010

using (SPSite site = new SPSite(SPContext.Current.Site.Url))
      {
        SPWebApplication webApp = site.WebApplication;
        foreach (SPSite sitecol in webApp.Sites)
        {
          SPWebCollection webs = sitecol.AllWebs;
          foreach (SPWeb web in webs)
          {
            SPUserCollection users = web.SiteUsers;
            foreach (SPUser user in users)
            {
              //code
            }
          }
        }
      }