This topic describes how to access Web services from client script in AJAX-enabled ASP.NET Web pages. The services can be either custom services that you create, or built-in application services. The application services are provided as part of ASP.NET AJAX, and include authentication, roles, and profile services.
Custom Web services can be in the form of .ASP.NET Web services (.asmx services), or Windows Communication Foundation (WCF) services (.svc services).
ASP.NET enables you to create Web services that can be accessed from client script in Web pages. The pages communicate with the server through a Web service communication layer that uses AJAX technology to make Web service calls. Data is exchanged asynchronously between client and server, typically in JSON format.
Client-Server Communication for AJAX Clients
In AJAX-enabled Web pages, the browser makes an initial request to the server for the page, and then makes subsequent asynchronous requests to Web services for data. The client communication elements are in the form of downloaded proxy classes from the server and the core client-script library. The server communication elements are handlers and custom services. The following illustration shows the elements that are involved in the communication between the client and the server.
AJAX Client Architecture
Browsers call Web service methods by using proxy classes. A proxy class is a script that is automatically generated by the server and downloaded to the browser at page-load time. The proxy class provides a client object that represents the exposed methods of a Web service.
To call a Web service method, client script calls the corresponding methods of the proxy class. The calls are made asynchronously, through the XMLHTTP object.
The Web service communication layer contains the library script types that enable the proxy classes to make service calls. For more information, see the classes that are contained in the Sys.Net namespace.
The code in the proxy classes and in the core Web service communication layer hides the complexity of XMLHTTP and the differences between browsers. This simplifies the client script that is required to call the Web service.
There are two approaches to making a Web service request:
Call Web services by using the HTTP POST verb. A POST request has a body that contains the data that the browser sends to the server. It does not have a size limitation. Therefore, you can use a POST request when the size of the data exceeds the intrinsic size limitation for a GET request. The client serializes the request into JSON format and sends it as POST data to the server. The server deserializes the JSON data into .NET Framework types and makes the actual Web service call. During the response, the server serializes the return values and passes them back to the client, which deserializes them into JavaScript objects for processing.
Call Web services by using the HTTP GET verb. This resembles the functionality of a POST request, with the following differences:
The client uses a query string to send the parameters to the server.
A GET request can call only a Web service method that is configured by using the ScriptMethodAttribute attribute.
Data size is limited to the URL length allowed by the browser.
AJAX Client Architecture
Elements of the client architecture include the Web service communication layer in the core library and the downloaded proxy classes for the services that are used on the page. Individual elements shown in the figure are as follows:
Custom Service Proxy Classes. These consist of client script that is automatically generated by the server and downloaded to the browser. The proxy classes provide an object for each WCF or ASMX service that is used in the page. (That is, they provide an object for each item in the ServiceReferences element of the ScriptManager control in the page.) Calling a proxy method in client script creates an asynchronous request to the corresponding Web service method on the server.
Authentication Proxy Class. The AuthenticationService proxy class is generated by the server authentication application service. It enables the user to log on or log out through JavaScript in the browser without making a round trip to the server.
Profile Proxy Class. The ProfileService class is generated by the server profile application service. It makes the current user's profile information available to the client through JavaScript without making round trips to the server.
Web Service Communication Layer. This is the library that contains the client script types. These types enable the browser (client) to communicate with the services on the server. They also shield client applications from the complexities of establishing and maintaining the asynchronous communication between client and server. They encapsulate the browser XMLHTTP object that provides the asynchronous capability, and enable client applications to be browser independent. The following are the main elements of the Web service communication layer:
WebRequest . This provides client script functionality to make a Web request. For more information, see the WebRequest class.
WebRequestManager . This manages the flow of the Web requests issued by the WebRequest object to the associated executor object. For more information, see the WebRequestManager class.
XmlHttpExecutor . This makes asynchronous network requests by using the browser's XMLHTTP support. For more information, see the XmlHttpExecutor class.
JSON Serialization. This serializes JavaScript objects into JSON format. Deserialization is available by using the JavaScript eval function. For more information, see the JavaScriptSerializer class.
The default serialization format is JSON, but individual methods in Web services and in ASP.NET Web pages can return alternative formats such as XML. The serialization format of a method can be specified with attributes. For example, for an ASMX service, you can set the ScriptMethodAttribute attribute to cause a Web service method to return XML data, as shown in the following example:
[ScriptMethod(ResponseFormat.Xml)]
AJAX Server Architecture
Elements of the server architecture include the Web service communication layer with a HTTP handler and serialization classes, custom services, page methods, and application services. Individual elements shown in the figure are as follows:
Custom Web Services. These provide the service functionality that you implement and return the appropriate response to the client. Custom Web services can be ASP.NET or WCF services. The Web service communication layer automatically generates client-script proxy classes that can be called asynchronously from client script.
Page Methods. This component enables a method in an ASP.NET page to be called as if it were a Web service method. Page methods must be defined in the page that is performing the page-method call.
Authentication Service. The authentication service generates an authentication proxy class that enables the user to log on or log out through client JavaScript. This application service is always available and you do not have to instantiate it. For more information, see Using Forms Authentication with ASP.NET AJAX.
Role Service. The role service generates a role proxy class that enables client JavaScript to access roles information for the currently authenticated user. This application service is always available and you do not have to instantiate it. For more information, see Using Roles Information with ASP.NET AJAX.
Profile Service. The profile service generates a profile proxy class that enables client JavaScript to get and set profile properties for the user that is associated with the current request. This application service is always available and you do not have to instantiate it. For more information, see Using Profile Information with ASP.NET AJAX.
JSON Serialization. The server JSON serialization component enables customizable serialization and deserialization of common .NET Framework types to and from JSON format. For more information, see JavaScriptSerializer.
XML Serialization. The Web service communication layer supports XML serialization for SOAP requests to Web services and for returning XML types from a JSON request to a Web service.
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