ObservableCollections is a high-performance .NET library providing generic observable collections (ObservableList, ObservableDictionary, ObservableHashSet, ObservableQueue, ObservableStack, ObservableRingBuffer, ObservableFixedSizeRingBuffer) with synchronized views, filters, and writable views. It replaces the non-generic, allocation-heavy .NET ObservableCollection with zero-allocation event args using ref structs and ReadOnlySpan. The library is designed for MVVM architectures in WPF, Blazor, Avalonia, Unity, and WinUI, offering both read-only and writable synchronized views that can be bound via INotifyCollectionChanged. It also provides R3 reactive extensions for Rx-style event observation. The codebase is highly optimized, leveraging modern C# features like ref structs, spans, and custom data structures (e.g., RingBuffer, AlternateIndexList) for minimal allocations and maximum performance.