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();  //获取用于写入请求数据的Stream对象
   }
   catch (Exception ex)
   {
    return "";
   }

   writer.Write(bytes, 0, bytes.Length);  //把参数数据写入请求数据流
   writer.Close();

   try
   {
    response = (HttpWebResponse)request.GetResponse();  //获得响应
   }
   catch (WebException ex)
   {
    return "";
   }

   #region 这种方式读取到的是一个返回的结果字符串
   Stream stream = response.GetResponseStream();  //获取响应流
   XmlTextReader Reader = new XmlTextReader(stream);
   Reader.MoveToContent();
   result = Reader.ReadInnerXml();
   #endregion

   #region 这种方式读取到的是一个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>