.NET Mobile Application Development XML Web Services

Introduction  In the previous session we examined Remote Procedure Call as a mechanism for allowing distributed software components to interact across the network  In this session we will examine >XML Web services, a simple form of cross-platform RPC >Creating and consuming Web services in.NET

XML Web Services  Web services >provide a simple form of cross-platform RPC >are objects hosted by Web server >enable interaction via textual XML messages >support use of user-defined types as arguments / return values >are not designed to be called by end users, only by other applications  Web services do not >offer a user interface >run continuously - objects created on demand (i.e. single call semantics) >preserve state across calls – stateless components

Web Service Fundamentals  Web services are based on open standards  XML >All interactions are textual messages  Web Services Description Language (WSDL) >XML description of Web service interface >Used by consumers to create proxies  Simple Object Access Protocol (SOAP) >XML encoding of data types >Used to serialize arguments and return values >Operates over http protocol  Web services are supported by many environments >e.g. Java,.NET, COM, etc

Web Service Architecture  Client uses WSDL to create proxy during development  At runtime, proxy used to interact with Web service  Proxy handles marshalling and other low-level details

Web Services and Serialization  Arguments / return values for Web services are serialized to XML .NET Web services use the XMLSerializer class to perform serialization. This class >provides simpler form of serialization than the full SOAP serialization used by.NET Remoting >only serializes public data members and simple-user defined types >does not transfer code with the serialized instance ­receiver cannot call methods, properties or events of this type

Supported Data Types in.NET Web Services  Basic types >bools, ints, floats, strings, dates & times  Enumerations  Arrays and simple collections >of supported types only  User-defined types >public data members and readable properties based on public members only  DataSet instances  XMLNode instances >portions of XML documents

Creating a.NET Web Service  Create dedicated IIS virtual directory >Use VS ASP.NET Web Service project type  Create C# class to implement service >Each remotely callable method must be tagged with [WebMethod] attribute >Simplest Web service is collection of stateless methods >Compiled to.asmx class  Create WSDL  Deploy WSDL and.asmx files to Web server  Everything in grey is done automatically by Visual Studio

Consuming a Web Service  Find Web service using known URL, discovery document or UDDI registry >‘Add Web Reference’ in Visual Studio  Retrieve WSDL for Web service >Used to create proxy before compilation >What happens if Web service interface changes?  Implement client logic and use proxy to invoke Web service  Everything in grey is done automatically by Visual Studio

Web Service Proxies  Created in.NET as hidden source code class in Web References folder  Contains >synchronous and asynchronous versions of Web methods defined by Web service >simple definitions of other types (public data members only) used, if needed  Bound to a specific Web service URL >Customizing the hidden source class constructor allows proxies to Web services of the same type to be constructed at runtime

Asynchronous Interaction  Web services offer two forms of asynchronous method invocation  One-Way Web service methods >Tagged with [OneWay] attribute >Call to one-way method via proxy returns immediately (fire-and-forget) >Return values are not possible; void methods only  Asynchronous method invocation >Standard feature of.NET >Simplified in Web services; generated proxy has asynchronous versions of all calls >Uses callback method to obtain return values

Example: Creating a.NET Web Service  Simple Web service to calculate cumulative interest  Single Web method required >Accepts two parameters ­Amount of loan (int) ­Length of loan (int) >Returns LoanDetails instance which contains the total amount payable ­Definition of LoanDetails type does not exist at client before proxy is built  Interest rate on loan is fixed and known only to Web service

Example: Consuming a Web Service  Simple Windows application for entering loan amount and period  Uses LoanCalculator Web service to retrieve LoanDetails instance with total loan amount  Note difference in members/properties in the LoanDetails type created by the proxy and that in the Web service.

 In this session we have discussed >XML Web services >Creating and using Web services in.NET  In the next session we will consider messaging and communication from mobile devices Summary

Reading and Resources Reading  MacDonald, Microsoft.NET Distributed Applications: Integrating XML Web Services and.NET Remoting, Microsoft Press, 2003 Chapter 5, pp139 – 174 Resources  Web Services Developer Centre >  Designing.NET Web Services > xhttp://msdn.microsoft.com/webservices/building/frameworkandstudio/designing/default.asp x  Indigo, the next generation of Web Services in Windows Longhorn >