How to get full host name + port number in Application_Start of Global.aspx?

I tried

Uri uri = HttpContext.Current.Request.Url;
String host = uri.Scheme + Uri.SchemeDelimiter + uri.Host + ":" + uri.Port;

and it worked well on my local machine, but when being published to IIS7, there is an exception saying

System.Web.HttpException: Request is not available in this context

Anyone know how to achieve this?