using System.Net;
           using System.Collections.Specialized;  


              //initialize 

            WebClient client = new WebClient();
            NameValueCollection values = new NameValueCollection();

            //parameters 

            values.Add("TPL_username", "admin");
            values.Add("TPL_password", "123");

            //submit   form 

            byte[] response = client.UploadValues("https://login.taobao.com/member/login.jhtml", "post", values);
            
            System.IO.FileStream f = System.IO.File.Create("d:\\temp.html");
            f.Write(response, 0, response.Length);
            f.Close();

------解决方案--------------------
最好调用TOPAPI  里面有接口