PInvoke and COM Interop in VS .NET

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PInvoke and COM Interop
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The situation is even more complicated if the Variant returned from COM has the VT_BYREF bit set, which indicates that the Variant contains a 32-bit pointer to the data. (For example, a Variant that contains a VT_BYREF+VT_DISPATCH value is actually a pointer to an object.) If the Variant is passed by value, the marshaler correctly recognizes the BT_BYREF bit and is able to retrieve the object reference, but any changes to this .NET object aren t propagated to the original COM object (unlike what would happen if the cli ent were a true COM application). If the Variant is passed with the ByRef keyword, however, changes to the .NET object are correctly propagated to the original COM object, but only if the COM method hasn t changed the type of the object. If the COM method has changed the object s type, an InvalidCastException occurs on return from the method. The bottom line: Steer clear of COM objects that take and return Variant arguments, if possible. If you can t avoid them, read the documentation carefully, and use the preceding notes as a guideline for troubleshooting code that doesn t behave as expected.
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One more issue must be taken into account when accessing COM from a .NET client. COM components live either in a Single Thread Apartment (STA) or in a Multi Thread Apartment (MTA), even though there are a few components that can live in both apart ment types. By comparison, .NET applications always run as free-threaded code, don t use apartments, and implement synchronization by other means (for example, syn chronized regions and locks). The .NET runtime must initialize either an STA or an MTA before a managed client can call a COM object. The type of the apartment being initialized affects all subsequent calls: if the apartment isn t compatible with the apartment where the COM component resides, COM has to create a proxy/stub pair between them, which in turn has a seri ous negative effect on performance. As you probably know, all Visual Basic 6 compo nents can live only in an STA; therefore, calls to these components coming from .NET applications that have initialized an MTA will be slowed by an intermediate proxy/stub. Visual Basic .NET console and Windows Forms applications initialize an STA by default, but other types of applications initialize an MTA by default. You can affect the apartment type that a managed thread creates by setting the Thread object s Apart mentState property, before the first call to COM is made:
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If Thread.CurrentThread.ApartmentState = ApartmentState.Unknown Then Apartment hasn t been created yet. Thread.CurrentThread.ApartmentState = ApartmentState.STA End If Now you can call a Visual Basic 6 component. Dim sc As New SampleComponent.SampleObject
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You can t change the apartment type after the thread has initialized the apartment. Fur ther attempts to modify the ApartmentState property are simply ignored, without raising any exception. Another way to tell .NET which apartment type should be created is by flagging the Sub Main procedure with either the STAThread or the MTAThread attribute:
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Part VII:
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Advanced Topics
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<STAThread()> _ Sub Main End Sub
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ASP.NET applications create MTA apartments by default, but you can set the Page.AspCompatMode property or the AspCompat attribute in an @Page directive to True to force the page to execute in an STA. This setting is also necessary to call COM+ objects that access built-in objects such as Request or Response either through the ObjectContext object or through the argument passed to the OnStartPage method. For this prop erty or attribute to work correctly, however, the COM or COM+ object must be created from inside the Page_Init or Page_Load event handler, as opposed to from inside the page s constructor or by means of a field initializer. Setting this property or attribute to True degrades the page s performance, so you should weigh the tradeoff of having an ASP.NET page running in an STA vs. going through a proxy/stub when the page calls an STA-only COM object.
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