Asynchronous Callback Procedures in Visual Studio .NET

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Asynchronous Callback Procedures
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As I ve already explained, the BeginInvoke method takes all the arguments in the orig inal method s signature, plus two additional arguments. The second-to-last argument is a delegate pointing to a callback procedure that s called when the asynchronous method completes its execution. This callback procedure offers a viable alternative to your making the main thread use the IsCompleted or AsyncWaitHandle property of the IAsyncResult object to determine when it s safe to gather the return value and any ByRef arguments. The callback procedure must follow the syntax of the AsyncCallback delegate (defined in the System namespace), which defines a Sub procedure that takes an IAsyncResult object as its only argument. The code inside the callback procedure should call the del egate s EndInvoke method to retrieve the return value and the value of any ByRef argu ments. Here s a possible implementation of the callback procedure for the example seen previously:
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Sub MethodCompleted(ByVal ar As IAsyncResult) Dim parsedDirs As Integer Dim files As ArrayList = findFilesDeleg.EndInvoke(parsedDirs, ar) Display found files. ...(Omitted, same as previous examples)... End Sub
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This approach poses two minor problems. First, the callback routine doesn t have any way to know why it has been called, so it s difficult to reuse the same callback routine for multiple asynchronous calls. Second, the delegate variable (findFilesDeleg in this particular example) must be visible to both the routine that makes the asynchronous call and the callback routine; this isn t a serious problem when both routines belong to the same class or module (you can just declare it as a private class-level variable), but it makes the approach rather clumsy when the callback routine is in another class, pos sibly located in a different assembly.
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Both problems can be solved if you take advantage of the last argument of the BeginInvoke method. This argument can be any object, so you can pass a number or a string that helps the callback routine understand which asynchronous method has just com pleted. More interestingly, you can pass an array that contains the original delegate object that you used to make the asynchronous call plus any other value required by the callback procedure. This approach helps you solve the second problem as well:
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Create a delegate that points to the target procedure.
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Dim findFilesDeleg As New FindFilesDelegate(AddressOf FindFiles)
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Create an array holding the delegate and other values
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Dim values() As Object = {findFilesDeleg, TXT files in D:\Docs"}
Start the async call, pass a delegate to the MethodCompleted
procedure and get an IAsyncResult object.
Dim parsedDirs As Integer
Dim ar As IasyncResult = findFilesDeleg.BeginInvoke( _
D:\docs", *.txt", parsedDirs, AddressOf MethodCompleted, values)
The callback method must extract data out of the values array, which is passed in the AsyncState property of the IAsyncResult object:
Sub MethodCompleted(ByVal ar As IAsyncResult) Extract data from the values array. Dim values() As Object = DirectCast(ar.AsyncState, Object()) Dim deleg As FindFilesDelegate = DirectCast(values(0), FindFilesDelegate) Dim msg As String = values(1).ToString Call the EndInvoke method, and display result. Dim parsedDirs As Integer Dim files As ArrayList = deleg.EndInvoke(parsedDirs, ar) For Each file As String In files Console.WriteLine(file) Next Console.WriteLine( {0} directories have been parsed.", parsedDirs) End Sub
More on Asynchronous Method Invocation
A relevant detail I haven t covered yet is how the asynchronous architecture deals with exceptions. It turns out that both the BeginInvoke and EndInvoke methods can throw an exception. If BeginInvoke throws an exception, you know that the asynchronous call hasn t been queued and you shouldn t call the EndInvoke method. These exceptions might be thrown by the .NET asynchronous infrastructure for example, when the target of the asynchronous call is a remote object that can t be reached. EndInvoke can throw an exception too; this happens either when the asynchronous method throws an exception or when the .NET asynchronous infrastructure throws an exception for example, when the remote object can t be reached any longer. The obvious suggestion is that you should bracket EndInvoke calls inside a Try End Try block, as you would do for any regular method call that can throw an exception.
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