The COM Course - Part 3

By Karl Moore

Compatibility

At the end of last week, we stumbled across a small problem. Well, rather a big problem actually.

In fact, if you've got time, it was a whopping great huge problem wearing high heels, a frilly skirt and holding a massive sign stating "I am a BIG problem!".

You see - we compiled our ActiveX DLL, then compiled a test program that used our DLL. Then we recompiled our DLL ?something you usually do after you make changes. Suddenly, our test program no longer worked. Damn.

If we had then recompiled our test program, it would've worked. But let's say your VB program, an excel spreadsheet and one C++ accounts package all use your DLL. Do they all need to be 'recompiled' every time you make a small change to your ActiveX DLL?

The answer is, thankfully, no.

The reason we experienced problems last week was due to an issue known as compatibility. Or rather, a lack of it.

• Open up your Northwind project in Visual Basic
• Click 'Project', 'Northwind Properties'
• Click the 'Component' tab

Take a peek at the 'Version Compatibility' frame in front of you. Here we have three options. Let's explain these now:

• No Compatibility ?With every compile, your COM component gets assigned a new 'signature'. This means programs looking for older 'signatures' (the previous version of the DLL) ?simply flop
• Project Compatibility ?With every compile, your COM component is assigned a new signature ?and still, any using-applications still flop. The only change here is that 'big' differences between your current project and a previous DLL project are highlighted as you compile. Ohhh, whoop-de-doo
• Binary Compatibility ?When compiled, your application attempts to keep the signature of a previously compiled DLL, thus ensuring any applications using it don't magically turn into the Blue Screen of Death. However if the differences between your previously compiled DLL and your to-be-compiled DLL are too great, a new signature must be assigned

So let's test our theory:

• Open last week's test application
• Recompile it
• Trial run your test application ?it should work
• Open our ActiveX DLL project
• Set the Binary Compatibility option
• Recompile your DLL
• Attempt to run your test application ?it should still work

Hurrah! It may sounds like complete hogwash, but when recompiling their DLLs, most developers get themselves into an awfully incompatible situation.

But hopefully you won't ?all thanks to this small aside.

Well, that's compatibility for you. About as exciting as www.PylonOfTheMonth.co.uk.

Introduction - Compatibility - ActiveX EXEs - Creating a Test ActiveX EXE - Instancing - Creating a Test Application - Distributing Components - Where to Go from Here - Conclusion -