public class WebServiceDemo : System.Web.Services.WebService
    {

        [WebMethod]
        public string HelloWorld()
        {
            return "Hello World";
        }

        [WebMethod]
        public string Sum(string param1, string param2)
        {
            int num1 = Convert.ToInt32(param1);
            int num2 = Convert.ToInt32(param2);

            int sum = num1 + num2;

            return sum.ToString();
        }
    }

class Program
    {
        static void Main(string[] args)
        {
            Program program = new Program();
            string url = "http://localhost:12544/WebServiceDemo.asmx";
            string method = "Sum";
            string num1 = "1";
            string num2 = "2";

            string result = program.HttpPostWebService(url, method, num1, num2);

            Console.WriteLine(result);
            Console.ReadKey();
        }

        public string HttpPostWebService(string url,string method,string num1,string num2)
        {
            string result = string.Empty;
            string param = string.Empty;
            byte[] bytes = null;

            Stream writer = null;
            HttpWebRequest request = null;
            HttpWebResponse response = null;

            param = HttpUtility.UrlEncode("param1") + "=" + HttpUtility.UrlEncode(num1) + "&" + HttpUtility.UrlEncode("param2") + "=" + HttpUtility.UrlEncode(num2);
            bytes = Encoding.UTF8.GetBytes(param);

            request = (HttpWebRequest)WebRequest.Create(url + "/" + method);
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            request.ContentLength = bytes.Length;

            try
            {
                writer = request.GetRequestStream();        //獲取用于寫入請求數據的Stream對象
            }
            catch (Exception ex)
            {
                return "";
            }

            writer.Write(bytes, 0, bytes.Length);       //把參數數據寫入請求數據流
            writer.Close();

            try
            {
                response = (HttpWebResponse)request.GetResponse();      //獲得響應
            }
            catch (WebException ex)
            {
                return "";
            }

            #region 這種方式讀取到的是一個返回的結果字符串
            Stream stream = response.GetResponseStream();        //獲取響應流
            XmlTextReader Reader = new XmlTextReader(stream);
            Reader.MoveToContent();
            result = Reader.ReadInnerXml();
            #endregion

            #region 這種方式讀取到的是一個Xml格式的字符串
            //StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.UTF8);
            //result = reader.ReadToEnd();
            #endregion 

            response.Dispose();
            response.Close();

            //reader.Close();
            //reader.Dispose();

            Reader.Dispose();
            Reader.Close();

            stream.Dispose();
            stream.Close();

            return result;
        }
    }

<system.web>
    <webServices>
      <protocols>
        <add name="HttpPost" />
      </protocols>
    </webServices>
  </system.web>