using System;
using System.Globalization;
using System.Web.Services;
using System.Web.Services.Protocols;

namespace WJSService
{
    /// <summary>
    /// 
    /// </summary>
    [WebService(Namespace = "http://www.heibing.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]

    public class SysMail : BaseService
    {

        /// <summary>
        /// 测试
        /// </summary>
        /// <returns></returns>
        [WebMethod(Description = "测试程序")]
        public string HelloWorld()
        {
            return "Hello World";
        }
    }
}

------解决方案--------------------
编译以后，更新客户端引用