Designing Queued Component Interfaces
When a client makes calls to a queued component, it interacts with the recorder provided by COM+. No actual calls to the real object occur. So, at the time of the call, the client has no way to receive output from the method, nor can it tell whether the call succeeded or failed. Consequently, when you design an interface to be supported by a COM+ queued component, you must avoid any outgoing parameters on any interface method.
Specifically, do not use any
attributes on your method parameters. When you import a component
into the Component Services Explorer, COM+ inspects
the interfaces supported by that component, and if it detects an
output attribute, COM+ disables the queuing option for that
If you develop your COM+ component using Visual Basic 6.0, you do not
have direct access to your component IDL. Normally, this lack of
access would not be a problem. However, Visual Basic, by default,
treats method parameters as
If you expect your component to be accessed as a queued component,
you have to explicitly use the Visual Basic
attribute on your method parameters.
In the next version of Visual Basic, Visual Basic.NET, all parameters are, by default, passed in by value instead of by reference. See Chapter 10 for more information.
A different kind of a parameter returned from a method is its return
value. You should avoid using custom
codes to indicate particular ...