When the address bar does not have the parameter "id":
Request.QueryString["ID"] == null
Convert.ToString(Request.QueryString["ID"]) == null
Note that this will go wrong:
Request.QueryString["ID"].ToString();
When the address bar has the parameter "id" but no value is assigned:
Request.QueryString["ID"] == ""
Request.QueryString["ID"] == String.Empty
Convert.ToString(Request.QueryString["ID"]) == ""
Convert.ToString(Request.QueryString["ID"]) == String.Empty
When judging two conditions at the same time:
if ("" + Request.QueryString["ID"] == "") {...}
if (("" + Request.QueryString["ID"]).Length == 0) {...}
Source: Jinshuiloutai BLOG