什么情况下HttpContext.Current.Request.UrlReferrer为空

The situations where this ServerVariable works include the following methods of a browser loading a URL: 

• clicking on a straight HTML <a href> link;
• submitting a form, using POST or GET, from a submit button, <input type=image> or client-side script (form.submit())

The situations where it doesn‘t work:

• clicking on a Favorite, History, or the recently-typed URLs list;
• clicking on ‘Home‘ in IE‘s toolbar, or an item in IE‘s ‘Links‘ toolbar;
• using location.href or location.replace() in client-side JScript/JavaScript/VBScript;
• using HierMenus (details);
• typing the URL directly in the browser and hitting Enter or clicking ‘Go‘;
• launching a clickable URL from an e-mail or MS Office document;
• using Response.Redirect / Server.Transfer;
• using Response.AddHeader (302) or <meta http-equiv=refresh> to redirect;
• loading the URL with XML (see Article #2173);
• misspelling the variable name (many people assume HTTP_REFERRER).