problem of use Request.Form.Allkeys

 I use vs2003 and trying to get all keys from my form here is my code

foreach( string ID in this.Page.Request.Form.AllKeys)
            {
                if(ID.Substring(0,3).Equals("ddl"))
                Response.Write(ID);

            }

but "this.Page.Request.Form.AllKeys" always obtain length 0
can you please tell me why??
does I use the wrong way? 
thanks

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