this.Request.Url.GetLeftPart(UriPartial.Authority)

可以获得http://www.baidu.com，无论是什么请求。另外，根据不同的UriPartial枚举值，可以得到path、QueryString等字符串。

一下内容来源：http://www.xueit.com/html/2009-03/21_932_00.html

---------------------------------------------------------------------

获取网站根目录的urli源代码

public static string GetRootURI()
    {
        string AppPath = "";
        HttpContext HttpCurrent = HttpContext.Current;
        HttpRequest Req;
        if (HttpCurrent != null)
        {
            Req = HttpCurrent.Request;

            string UrlAuthority = Req.Url.GetLeftPart(UriPartial.Authority);
            if (Req.ApplicationPath == null || Req.ApplicationPath == "/")
                //直接安装在   Web   站点   
                AppPath = UrlAuthority;
            else
                //安装在虚拟子目录下   
                AppPath = UrlAuthority + Req.ApplicationPath;
        }
        return AppPath;
    }

本文转自齐师傅博客园博客，原文链接：http://www.cnblogs.com/youring2/archive/2011/09/15/2177068.html，如需转载请自行联系原作者