PropertyChanged.Fody is a Fody add-in (weaver) and Roslyn source generator that automates the implementation of INotifyPropertyChanged (and optionally INotifyPropertyChanging) for .NET classes. The weaver operates on compiled IL using Mono.Cecil, injecting property change notification code into property setters, while the optional source generator emits boilerplate for partial classes at compile time. The tool supports multiple invocation patterns (event, OnPropertyChanged methods, before/after callbacks), attribute-driven dependency detection ([DependsOn], [AlsoNotifyFor]), equality checks before notifications, and extensible interceptors. Configuration is done via FodyWeavers.xml attributes or custom attributes on classes/properties. The project is mature, well-tested, and widely used (nearly 2000 stars), with extensive test coverage across inheritance, generics, nullable types, F#, and integration with MVVM frameworks like Caliburn.Micro and ReactiveUI.