Asynchronous Method Calls in .NET framework

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Asynchronous Method Calls
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As I ve shown you before, the proxy class exposes a Beginxxxx and Endxxxx pair of methods for each procedure in the Web service component marked with the WebMethod attribute. You can use these two methods for calling the Web service asynchronously.
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The actions necessary to invoke a Web service method asynchronously are identical to the actions needed to invoke an asynchronous delegate. I covered asynchronous del egates in 12, so in this section I ll give you just an example of asynchronous invocation of a Web service method.
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Sub CallAsyncMethod() Dim service As New localhost.SampleService Call a lengthy method that takes about 5 seconds to complete. Note that the proxy object is passed in the third argument. Dim ar As IAsyncResult = service.BeginLengthyMethodCall(5, _ AddressOf MethodCallback, service) End Sub This is the callback method. Sub MethodCallback(ByVal ar As IAsyncResult) Retrieve the proxy object from the AsyncState property. Dim service As localhost.SampleService = _ DirectCast(ar.AsyncState, localhost.SampleService) Complete the method call. service.EndLengthyMethodCall(ar) End Sub
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Aborting a Web service asynchronous call is slightly more difficult than aborting a syn chronous call. You must cast the IAsyncResult object returned by the Beginxxxx method to a WebClientAsyncResult object and then call the Abort method of the WebClientAsyncResult object:
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Run a method that takes 5 seconds to complete.
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Dim service As New localhost.SampleService()
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Dim ar As IAsyncResult = service.BeginLengthyMethodCall(5, _
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Nothing, Nothing) Do something else here. If ar.IsCompleted Then If the method completed, complete the call. service.EndLengthyMethodCall(ar) Else Else, cast to WebClientAsyncResult to abort the call. DirectCast(ar, WebClientAsyncResult).Abort() End If
One-Way Methods
A special case of asynchronous calls occurs when the client doesn t really care about the return value from the Web service. For example, you might build a Web service that exposes one or more methods that clients call only to signal that something has occurred or to issue commands (for example, to start a lengthy batch compilation). In such a case, you can reach perfect asynchronicity by flagging the Web service method using a SoapDocumentMethod attribute with its OneWay argument set to True:
Part VI:
Internet Applications
<WebMethod(), SoapDocumentMethod(OneWay:=True)> _ Sub OneWayLengthyMethodCall(ByVal seconds As Integer) Simulate a lengthy method. Thread.Sleep(seconds * 1000) End Sub
Methods flagged in this way must have neither a return value nor ByRef arguments. Also, they can t access their HttpContext object, and any property of the Web service class returns Nothing.
Working with Proxy Servers
If the client is behind a proxy server, calls to a Web service might fail. In most cases, you can solve this problem by creating a System.Net.WebProxy object and assigning it to the Proxy property of the client s proxy object:
Dim service As New localhost.SampleService
True means that we want to bypass the proxy for local addresses.
service.Proxy = New WebProxy( http://proxyserver:80", True)
service.LengthyMethodCall(10)
The CookieContainer Property
Remember from the section Enabling Session State, earlier in this chapter, the stan dard proxy class doesn t work well with Web service methods that rely on session state that is, methods whose EnableSession attribute is set to True because the proxy class can t work as a cookie container. Therefore, it can t store the session cookie that ASP.NET sends to the client when a new session is detected. Fortunately, making the proxy class a valid cookie container is just a matter of ensur ing that its CookieContainer property holds a reference to a System.Net.CookieCon tainer object:
Dim service As New localhost.SampleService() Each time this method is called, the value in the Label control is incremented by 1. Sub TestIncrementCounterMethod() Make the proxy object a cookie container, if necessary. If service.CookieContainer Is Nothing Then service.CookieContainer = New CookieContainer() End If Label1.Text = service.IncrementCounter() End Sub
After setting the CookieContainer property, the proxy object is able to store the session cookie. The session is terminated when the proxy object is set to Nothing or when the session timeout expires without invoking any method in the Web service. (The default session timeout is 20 minutes.)
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