Web Services in .NET framework

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Web Services
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However, be prepared for an unpleasant surprise when you call the method from a Windows Forms application that uses a proxy class. In this case, the IncrementCounter method always returns 1, and the GetSessionID method returns a different string at each invocation. To explain this odd behavior, you must remember that session support depends on a nonpersistent cookie sent to the client the first time the client sends a request to the ASP.NET application. If the client is a browser, the session cookie is preserved in the browser s memory and resubmitted at each subsequent request so that the application can recognize the client and associate it with the proper set of session variables. If the client is a Windows Forms application, however, the session cookie can t be stored anywhere, and each new request appears as if it were the first request coming from that client. In the section The CookieContainer Property, later in this chapter, you ll see how to work around this issue.
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Building Transactional Web Services
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You can use the TransactionOption argument of the WebMethod attribute to create transactional Web services, which can call transactional COM+ components and take advantage of Microsoft Distributed Transaction Coordinator (MS DTC) support. (You can read more about transactional components in 31.) Due to the stateless nature of Web services, a Web service method can be only the root of a transaction. In other words, you can t start a transaction in a client which might be a Windows Forms application, a Web Forms page, or another Web service method and propagate the transaction through a call to the Web service. The TransactionOption argument can take five different values Disabled, NotSup ported, Supported, Required, and RequiresNew but because of the limitation I just mentioned, only two different behaviors are available. You have a transactional Web service if you specify Required or RequiresNew, and a regular, nontransactional Web service if you use any other value for this argument. Here s the typical structure of a transactional Web service method:
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<WebMethod(TransactionOption:=TransactionOption.RequiresNew)> _ Function UpdateDatabase() As Boolean Dim cn As New System.Data.OleDb.OleDbConnection(myConnString) Try cn.Open() Perform all the required update operations. If everything went well, commit the transaction. ContextUtil.SetComplete() Let the client know that everything is OK. Return True Catch If an update error occurred, abort the transaction. ContextUtil.SetAbort() Let the client know that something went wrong. Return False
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Part VI:
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Finally Close the connection in all cases. cn.Close() End Try End Function
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You can also flag the method with the AutoComplete attribute, in which case the transaction is rolled back if the method throws an exception but committed otherwise, so you can omit the call to the SetComplete and SetAbort methods.
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The Web Service Proxy Class
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By default, Visual Studio .NET hides the proxy class that it creates. To make it visible, you must click on the Show All Files button on the Solution Explorer window s toolbar, expand the Web References element, and then expand the Reference.map element. In addition to the specific methods that mirror those that the Web server exposes, the proxy class inherits several useful properties and methods from the System.Web.Proto cols.SoapHttpClientProtocol class. For example, you can use the Timeout and Url prop erties to invoke a different Web service with the same interface, should the default Web service be unavailable. UserAgent is the user agent header used in the request to the Web service; the default is Mozilla/4.0 (compatible; MSIE 6.0; MS Web Services Client Protocol w.x.yyyy.z, where w.x.yyyy.z is the version of the common language runtime). Two properties have to do with user credentials. If the PreAuthenticate property is True, all the requests must contain authentication credentials; if it is False (the default), the credentials are sent only if the Web service disallows anonymous access and returns a 401 HTTP return code when the request is initially attempted. If AllowAu toRedirect is False (the default), the proxy class throws an exception if a server redi rection is attempted; this setting is appropriate if the message contains authentication information or other confidential information and a redirection to another server would compromise security.
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