jcfr/Slicer — comment on commit 091329e31b8ff38a3cb71f4b81f004d612cb6147
File: Base/QTCore/qSlicerExtensionsManagerModel.h line 40
By finetjul:

Semantically, the property is "installedExtensions", not its count.
In the same way, a car has a "color" property, not a "red" property, a "green" property and a "blue" property.
or
a parent HAS children, it doesn't HAVE childrenCount.