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issue If you subclass with events and show a MsgBox or modal form while debugging in the IDE, all the events in the modally-blocked part of the application stop firing If you are subclassing with event procedures, you don't get window message notifications, and your control doesn't function properly This is unacceptable for a commercial-quality control, especially when it doesn't have to happen Frozen events are generally not a problem in a compiled application Controls that use an Implements-interface callback instead of an event procedure are a big improvement, but they still involve controlinstantiation, the lookup costs for every message, and vtable-call overhead for the relevant message This does not compare favorably with SubClassBas SubClassBas is compiled into your project You needn't rely on external files No one needs to know which technologies you're using to make your application tick You already own SubClassBas If you can't figure out the API work necessary for subclassing, you are kidding yourself if you think you'll be able to do any meaningful API work within your window procedure This is not meant to discourage you from using subclassing: I'm just pointing out that subclassing is one of the easiest things you can do with the Win32 API Debugging Subclassed Windows Despite the disadvantages inherent in subclassing controls, there is one legitimate perceived advantage: If you subclass using a control, you can actually hit a breakpoint while debugging The problem with subclassing directly to an AddressOf procedure is that VB doesn't actually give you a function pointer to the code itself, but rather to a small thunk procedure This procedure prevents you from running the AddressOf function when you're in break mode This thunk is out of the picture after you compile your executable, but it wreaks havoc when you're in the IDE VB doesn't actually crash if you enter break mode while a direct subclass is in place Instead, you confuse Windows just enough that you can't use either the
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IDE or the form you're trying to debug Windows requires that a window procedure take certain actions before it returns control, but the AddressOf blocking thunks do nothing and return 0 If you're lucky, you'll be able to hit F5 and continue But if not, you have to kill the VB process and start over Fortunately, there is a way out of this situation The solution is built into SubClassBas The approach is straightforward: If VB is in break mode, you defer to the original window procedure directly instead of calling the AddressOf-supplied function first In break mode, the project will run as if the subclass didn't exist A DLL, DbgWProcDII, makes this possible DbgWProc was first written for the February 1997 article I mentioned earlier, and it is also mentioned in MSDN as an available download The original version supports only 100 concurrent subclasses, but the version with the book uses the PushParamThunk code to remove this limitation DbgWProc is easy to use, and it is designed to add no additional runtime overhead to subclassing code Take the following steps to get painless debugging with direct API subclassing 1 Add a project reference to Debug Object for AddressOf Subclassing If this entry is not available in the References dialog, Browse to the PowerVB\Tools directory to add the reference 2 On the Project Properties Dialog's Make tab, add the conditional compilation value DEBUGWINDOWPROC = 1 3 Use SubClass and UnSubClass from SubClassBas just as you did before 4 Change the DEBUGWINDOWPROC conditional value to 0 before you choose File/Make Exe If you fail to do this and load DbgWProcDII when the IDE is not loaded, then you'll get a message box that says, "WindowProc debugging isn't required outside the Visual Basic design environment" This is a benign message (you can continue running the application), but you probably don't want to show it to your customers You should not distribute DbgWProcDII with your application You can refer to SubClassBas on the CD for the code that enables debugging with an active subclass Of course, SubClassBas does all the work for you,
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