how to use webservice in c#

AuthorityElement objElement = new AuthorityElement();

        objElement.Org = "mitl";

        objElement.User = "xml";

        objElement.Password = "xmltest";

        objElement.Currency = "USD";

        objElement.TestDebug = true;

        objElement.Version = "1.17";

        AdultElement adult = new AdultElement();

        //adult[0].Id = 0;

        adult.Id = 0;

        //adult[0].Surname = "test";

        adult.Surname = "test";

        //adult[0].Title = "Mr.";

        adult.Title = "Mr.";

        //adult[0].Forename = "test";

        adult.Forename = "test";

        HotelStayDetailsRoomElement obj_room_details = new HotelStayDetailsRoomElement();

        GuestElement[] obj_guest = new GuestElement[1];

        obj_guest[0] = adult;

        obj_room_details.Guests = obj_guest;

        obj_room_details.BasicRoomType = "";

        obj_room_details.MealType = "";

        HotelStayDetailsElement obj_hotalstay = new HotelStayDetailsElement();

        obj_hotalstay.ArrivalDate = Convert.ToDateTime("2013-07-29");

        obj_hotalstay.Nights = 2;

        obj_hotalstay.Nationality = "AE";

        obj_hotalstay.Room[0] = obj_room_details;

        //HotelSearchCriteriaElement obj_search_criteria = new HotelSearchCriteriaElement();

        //obj_search_criteria.MinStars = 0;

        //obj_search_criteria.MinPrice = 0;

        //obj_search_criteria.MaxPrice = 0;

        //obj_search_criteria.AvailabilityStatus = EnumAvailabilityStatus.any;

        AvailabilitySearch objSearch = new AvailabilitySearch();

        objSearch.Authority = objElement;

        objSearch.RegionId = 19793;

        objSearch.HotelId = 0;

        objSearch.DetailLevel = EnumDetailLevel.basic;

        objSearch.CustomDetailLevel = "basic,canxfees";

        objSearch.MaxResultsPerHotel = 0;

        objSearch.MaxHotels = 0;

        objSearch.MaxSearchTime = 0;

        objSearch.HotelStayDetails = obj_hotalstay;

        //objSearch.HotelSearchCriteria = obj_search_criteria;

        ServiceSoapClient objClient = new ServiceSoapClient();

        AvailabilitySearchResult arryAvSeach = objClient.AvailabilitySearch(objSearch);


I am getting a error on this webservice
http://roomsxmldemo.com/RXLStagingServicesV212/ASMX/XmlService.asmx


when you run this code you getting a error 
"Object reference not set to an instance of an object."

so guys plz help me i have spend two days to find how to set code and call webservice 
thanks