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Figure 30-4
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The .NET Framework creates an RCW not only when a managed client explicitly instantiates a COM object, but also when a COM method returns a reference to a new
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object (more precisely, a reference to a COM object that isn t already known to the .NET Framework). Object identity works also in this case because the COM Interop infrastructure always creates one distinct RCW for each COM object accessed from .NET, regardless of whether it was instantiated by your code or returned by another method. The COM Interop infrastructure correctly preserves identity even in the most intricate cases. For example, when a managed client passes a .NET object as an argu ment to a COM method and the unmanaged code later returns that object to .NET, COM Interop correctly recognizes that the returned object is actually a managed object and returns a reference to that managed object, without creating an RCW. A .NET client can create a COM object in two different ways: as an early-bound object or as a late-bound object. You usually create an early-bound object by calling its constructor (with New) and assigning the result to a specific variable; you create a late-bound object by using Activator.CreateInstance and assigning the result to a generic Object variable. The difference in how the object is created affects the type of the RCW object that COM Interop creates behind the scenes. In the early-binding case, the RCW is a well-defined .NET type, and you can invoke its methods using the dot syntax, as you do with all .NET types. In the late-binding case, the type of the RCW is System.__ComObject, and you can invoke the methods of the wrapped COM object only via reflection (including reflection methods that Visual Basic transparently calls for you when Option Strict is off and you use late-bound calls). The .NET runtime can early-bind a COM component only if metadata is available for it. This metadata must be available both at compile time (to generate the IL code) and at run time (to create the RCW object). The metadata for a COM component is similar to the metadata associated with standard .NET types, except for a few attributes that iden tify that type as a COM component. In general, this metadata is extracted from the COM object s type library, implicitly if you use Visual Studio.NET or explicitly if you use the TlbImp utility.
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Because this is a book on .NET programming, I am not going to assume that you are familiar with basic COM-related concepts, such as interface programming and type librar ies. You can refresh your memory by reading 16 of my book Programming Microsoft Visual Basic 6, provided on the companion CD. That chapter includes a tutorial on COM com ponent creation using Visual Basic 6 and can prove useful when you re creating test COM components for use from managed applications.
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See Also
Importing a COM Component with Visual Studio .NET
The simplest way to create metadata for a COM component is to let Visual Studio.NET do it for you. Right-click on the References node in the Solution Explorer window (or choose the Add Reference command from the Project menu) to display the Add Refer ence dialog box, and then click the COM tab. It might take several seconds to fill this
Part VII:
Advanced Topics
window the first time you run this command because Visual Studio parses the system registry and looks for all the registered COM components. When this process is com pleted, you see a list of components like the one shown in Figure 30-5. In the next sec tion I will describe a code sample that uses COM Interop to access Microsoft Word, so you ll need to add a reference to the Microsoft Word library to compile the sample. Select the library from the list and click OK to start the type library import process. When the dialog box disappears, you will see new nodes in the References subtree in the Solution Explorer window; you can right-click on them to display their Properties.
Figure 30-5
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