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Binary Compatibility

I have an ActiveX dll I created that is in use by several clients.  I need to add some additional functionality that requires me to pass an additional piece of info to the component.

This may be a dumb question, but I will ask anyways.  Should I add the new parameter as a public property, or should I just add it as a parameter you have to pass to a method.  In otherwords, "Public Function MyMethod(ByVal MyNewParameter as String) As String".

Will either of these maintain binary compatibility?

1 Solution
Guy Hengel [angelIII / a3]Billing EngineerCommented:
If you ADD a property, binary compatibility will be maintained.
If you ADD a parameter, binary compatibility will break.
If you ADD a new function (MyFunctionEx), with the same parameters + the new one, compatibility will be maintained.

Depending on the usage, I take different choice. Most of the cases, I use both, adding the property to call the old function afterwards, or if the old function is called, it will in fact run the new function with some default value for this new parameter.

When you add new Public Property what you will be doing is basically enhancing your component. As long as you make no changes to existing properties and methods, Visual Basic updates the version number of the type library but keeps it compatible with the old version number. Client applications that are built using the new version of your component will compile with the new version number, and can make use of all the new features. They cannot be used with earlier versions of your component, however, because type library versions are only upward-compatible.

Adding a parameter to the Method will cause VB to make the version number of the type library incompatible, rendering the new version unusable with client applications compiled with earlier versions.

d_glennAuthor Commented:
Thanks for both your help.  Exactly what I am looking for.  Since you both answered the question at the same time, you both deserve the points.

rpai, you can find your points here:

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