A reader of a previous post on binary compatibility asked for clarification about my claim about VB Migration Partner being the the only conversion tool on the market offering this important feature, mentioning that at least another tool from our competitors also claim to offer this feature, which they obtain by adding ComVisible, ProgID, and ClassInterface attributes.
I am aware there is some margin for ambiguity and for uncorrect information, so I decided to drill a bit deeper on this topic.
You have binary compatibility between a COM component a .NET component if you can unregister the COM component and install the .NET component in its place, without having to recompile the COM clients that were using the COM component. We call it "binary compatibility" because it's the kind of compatibility we used to enforce under VB6, to ensure that a new version of a given component was perfectly compatible with its previous releases.
The ability to generate a VB.NET component that is binary-compatible with the original VB6 component is greatly useful, for many reasons:
- You can migrate a complex N-tier application starting with its innermost DLLs, typically those that access the database, without having to recompile or modify in any way existing COM clients.
- The ability to convert only a subset of existing DLLs without being forced to migrate the entire application allows you to focus on those components that suffer most from the inherent scalability and performance limitations imposed by COM. This is the so-called phased migration.
- Just as important, you can immediately debug the converted DLLs because you can use existing COM clients and their user interface - no need to write small throw-away .NET projects with the only purpose of testing the DLLs you just converted.
- Finally, if your organization is very serious about testing, odds are that you have defined unit tests, use cases, and maybe scripts that rely on commercial testing tools. The great news is that, in most cases, binary compatibility allows you to test the converted DLLs using existing unit tests and scripts, thus saving you even more time.
As you see, binary compatibility can be a tremendous time-saver when migrating a VB6 application that is split in many DLLs, and it's no surprise that a migration tool vendor would like to claim that its product offers this feature.
Unfortunately for our competitors, to generate a .NET class that is binary-compatible with the original VB6 class you need much more than three attributes. You have to extract GUIDs from the original DLL and apply them correctly to the generated .NET class, you must handle fields, enums, types, events, and interfaces with special care, and you have to pay attention to many special cases and exceptions.
More precisely if you just add the ComVisible, ProgID, and ClassInterface attributes, the best you can get is
compatibility with existing VBScript and ASP clients, not with
compiled COM clients
The bottom line: based on what I can read and what I heard from customers, I can only reinforce my claim that only VB Migration Partner offers true backward binary-compatibility with existing COM clients and fully supports phased migrations of N-tier VB6 apps.