[ServiceContract(Namespace = "WcfService1")]
    public interface IService1
    {
        [OperationContract]
        string CheckUserLogin(string Username, string Password);
    }

    public class Service1 : IService1
    {
       // [WebInvoke(Method="POST",RequestFormat = WebMessageFormat.Json)]
        public string CheckUserLogin(string Username, string Password)
        {

            if (string.IsNullOrEmpty(Username) || string.IsNullOrEmpty(Password))
                return "用户名或密码不能为空。";

            if (Username == "error")
                throw new ArgumentException("无效的用户名。");

            if (string.Compare(Username, "fish", true) != 0 || string.Compare(Password, "123456", true) != 0)
                return "用户名或密码不正确。";

            return "OK";
        }
    }