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Working with ASP.NET Web Services
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calls, the Web Service isn t aware that you d like to include this class in the proxies. To enlighten the Web Service about your intentions, you can leverage the GenerateScriptType tag. If you apply this tag to the Web Service class, along with the type of class you d like to include, it too will be supported in the web service proxy. Listing 5.9 shows how the Web Service class is updated with the script-type declaration of the Employee class.
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Listing 5.9 Adding support for the Employee class on the client using GenerateScriptType
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[ScriptService] [GenerateScriptType(typeof(Employee))] [WebService(Namespace = "http://aspnetajaxinaction.com/")] [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)] public class StarbucksService : System.Web.Services.WebService { ...
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This is all you need to do to provide support for instantiating a server-side object on the client. To prove that this class can be created and manipulated from JavaScript, add the following lines of markup and script to the page:
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<hr /> <div> <input id="CreateEmployee" type="button" value="Instantiate Employee" onclick="createEmployee()" /> </div> ... function createEmployee(){ var emp1 = new AspNetAjaxInAction.Employee(); emp1.First = "Frank"; emp1.Last = "Rizzo"; emp1.Title = "Principal"; }
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Without the GenerateScriptType annotation in the Web Service class, a runtime exception would occur when you try to instantiate the Employee object. Instead, you re able to create an instance and update its properties accordingly. Making asynchronous requests from JavaScript to a Web Service is pretty easy. What isn t as easy is changing the way these request are submitted. Let s take a closer look at the types of requests we re talking about.
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Making asynchronous network calls
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Using HTTP GET
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So far, all the calls you ve made to the Web Service have used the HTTP POST verb. As a security measure, which we ll delve into in a minute, ASP.NET AJAX accepts these types of requests only from the browser by default. To accommodate an HTTP GET request, you re forced to explicitly adorn a method with the ScriptMethod attribute as well as set its UseHttpGet property to true. This subtle but conscious declaration prevents you from inadvertently letting the browser invoke methods with the HTTP GET verb. Listing 5.10 demonstrates how to update one of the existing methods, GetDeals, with HTTP GET capabilities.
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Listing 5.10 Enabling HTTP GET by updating the ScriptMethod attribute and UseHttpGet property
[ScriptMethod(UseHttpGet=true)] [WebMethod] public List<Beverage> GetDeals() { ...
What s all the fuss about Why is HTTP GET disabled by default The primary reason is to avoid compromising security in Ajax applications. To help you understand the kind of security we re talking about, we ll describe how JSON hijacking works. A common approach for JSON hijacking is to introduce into a page a malicious script that invokes an HTTP GET request, like so:
<script type="text/javascript" src="someReallyEvilScript.js"> </script>
Because the script is included on the page, it evades the origin policy that browsers enforce. This policy is put in place to limit objects like XMLHttpRequest from calling URLs in the same domain. This exploit leaves the JSON payload open for viewing and manipulation of the script. Thankfully, the ASP.NET AJAX framework provides more than one barrier for stopping this problem (a technique known as security in depth). The first layer of security for this scenario forces you to explicitly enable HTTP GET on a method, as we just covered. Second, validation against the Content-Type header field of the request is applied to ensure that it s set to application/json. It s interesting to note that when browsers parse external scripts that are included on a page, the content type is never set to application/json when making the
Working with ASP.NET Web Services
request. If any of these conditions aren t met (HTTP GET settings or the application/json content type), then the request is promptly rejected. Before we wrap up this section on working with Web Services, we ll explore one more approach. It involves making JavaScript calls to methods on a page, instead of to a Web Service.
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