Advanced Topics in Visual Studio .NET

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Advanced Topics
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Note that the .NET Framework must be installed on client computers to access serviced components remotely, regardless of whether the client application is managed or unmanaged. In additional, Service Pack 3 is also required for Windows 2000 clients. The installation process installs all the involved assemblies in the client s GAC, but these assemblies aren t removed from the GAC if you later uninstall the application. The managed assemblies are installed on the client because the client must access the assembly s metadata for example, to read the value of the AutoComplete attribute. This implies that the component might behave incorrectly if you change the assembly on the server without re-exporting it to all clients.
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Serviced Components Up Close
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Now that you are familiar with the basics of COM+ programming under .NET, you re ready to dive into more advanced topics, such as interacting with the call context via code and leveraging COM+ 1.5 features.
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Classes and Interfaces
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Most of the techniques I ve described so far are based on attributes, but a good pro grammer should be able to code against the objects and the interfaces that COM+ pro vides. In this section, I ll illustrate some of these techniques.
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The ServicedComponent Class
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The author of a .NET serviced component can interact with some of the features COM+ exposes by overriding protected methods in the ServicedComponent base class. The two most important of these methods are Activate and Deactivate, which fire when a JIT-activated object is brought to life and terminated:
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Protected Overrides Sub Activate() Initialize the object here. End Sub Protected Overrides Sub Deactivate() Clean up the object here. End Sub
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The Activate method fires immediately after the Construct method (if the ConstructionEnabled attribute is present), whereas the Deactivate method fires immediately before the Dispose protected method:
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Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean) End Sub
31:
Serviced Components
Unlike normal .NET objects which are released when the garbage collector collects them JIT-activated objects are disposed of automatically by the .NET runtime when the object is deactivated. You can therefore use the Dispose protected method as a sort of deterministic finalizer for serviced components. (You can create an explicit Finalize method, but you would incur an unnecessary performance penalty in doing so.) Clients should explicitly dispose of objects that aren t JIT-activated. The suggested way to do so is invoking the DisposeObject shared method of the ServicedComponent class:
In the client application ServicedComponent.DisposeObject(obj)
The ContextUtil Class
You ve already seen how to use the ContextUtil object to programmatically commit or abort a transaction, or to determine whether the caller is in a given role. This object exposes several other properties and methods of interest. All its members are shared. Most of the shared properties of this class are read-only and return a System.Guid value: ApplicationId (the current application), TransactionId (the current transaction), ActivityId (the current activity), ContextId (the current context), PartitionId (the current partition), and ApplicationInstanceId (the current application s instance). All the prop erties in this group are useful mostly for reporting purposes for example, to easily identify a specific transaction in the list of running transactions in the MMC snap-in. The PartitionId and ApplicationInstanceId properties are supported only under Win dows XP and Windows Server 2003. The IsInTransaction read-only property returns True if the object is running in a transaction. This property is useful for objects that support transactions but don t require them. Such objects should vote for the transaction outcome only if necessary:
If ContextUtil.IsInTransaction Then ContextUtil.SetComplete() End If
The ContextUtil class lets you read and set the done bit and the consistent bit that all COM+ objects manage. (See the section Defining a Transactional Class, earlier in this chapter.) You already know that the SetComplete method sets both these bits to True (the transaction can be committed and the object can be deactivated), whereas the SetAbort method sets the done bit to True and the consistent bit to False (the transaction is doomed). You can also vote for a transaction outcome without deactivating the object, by means of the EnableCommit method (which sets the consistent bit to True and the done bit to False) and the DisableCommit method (which sets both bits to False); these methods can be useful if the client can perform multiple calls into the object to complete the task at hand.
Part VII:
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