In one of my last blog posts I wrote about how we would be changing the file extension of WiX extensions from .dll to .wixext. Well, it turns out that we won't be doing it for technical reasons.
A WiX extension is really just a .NET assembly with an embedded XSD and wixlib. While it's technically possible to rename a managed assembly from .dll to .wixext, it will add a limitation that we didn't want to enforce. If WixExtensionA.wixext had a runtime reference to WixExtensionB.wixext, then the runtime cannot find the file if it doesn't end in a .dll. I personally think that's a limitation that the .NET Framework did not need to impose, but I also do not know all of the details. I'm sure there was a good reason to impose that limitation. We also had some problems with Visual Studio not letting us add project references to other WixExtensions that didn't end in a .dll.
As a result, we will not be doing this change and the current behavior will remain.