All you need is an .asmx file and the [WebMethod] attribute to define your
Web service:

Your asmx file can look like this:

<%@ WebService Language="C#" Class="Util" %>
   using System;
   using System.Web.Services;
   public class Util: WebService
    {
      public int Add(int a, int b)
      {
         return a + b;
      }
   
      [ WebMethod]
      public long Multiply(int a, int b)
      {
         return a * b;
      }
  }

If you want to store your code inside a .cs file, then bring it out of your
asmx file.  Then, compile the .cs file as a dll and put both the asmx file
onto IIS application and the dll inside the bin/ directory of your
application.