How to make public properties(get;set) in asp.net webservice and access at client end.

public class Service1 : System.Web.Services.WebService
    {
        private static string _NPI=string.Empty;
        public static string NPI
        {
            get
            {
                return _NPI;
            }
            set
            {
                if (!string.IsNullOrEmpty(value))
                {
                    _NPI = value;
                }
            }
        }

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

        [WebMethod]
        public int Add(int a,int b)
        {
            return a + b;
        }

        [WebMethod]
        public int Min(int a, int b)
        {
            return a - b;
        }
    }

ServiceReference1.Service1SoapClient obj = new ServiceReference1.Service1SoapClient();
                int a = obj.Add(5, 8);
                a = obj.Min(a, 10);
            //Methods are accessible but why not i am able to get,set property.
            //Please help?