Using ASP to judge the parameter of IE address bar

In the server-side ASP program can accept the HTML page on the form of the parameters, then how does it achieve ie address parameter judgment?

When the address bar has no parameter "id":

以下是引用片段：
Request.QueryString["ID"]==null
Convert.ToString(Request.QueryString["ID"])==null

Note that this can be an error:

request.querystring["ID"]. ToString ();

When the address bar has a parameter "id" but no value is assigned:

以下是引用片段：
Request.QueryString["ID"]==""
Request.QueryString["ID"]==String.Empty
Convert.ToString(Request.QueryString["ID"])==""
Convert.ToString(Request.QueryString["ID"])==String.Empty

At the same time judge two conditions:

以下是引用片段：
if(""+Request.QueryString["ID"]==""){...}
if((""+Request.QueryString["ID"]).Length==0){...}