This repository provides a WPF custom control library, Sdl.MultiSelectComboBox, which is a fully-featured multi-select combo box with support for item selection, grouping, sorting, filtering, autocomplete, and suggestion providers. The library includes a set of well-defined interfaces (IFilterService, IAutoCompleteService, ISuggestionProvider, IItemGroup, IItemEnabledAware) that allow developers to plug in custom logic. The main control class (MultiSelectComboBox) is a large, self-contained WPF custom control that handles all UI interactions, including keyboard and mouse input, popup management, and visual state management. The codebase also includes a reusable event behaviour factory for attaching commands to routed events, visual tree helper utilities, and a theme manager for dynamic resource switching. The example project demonstrates a realistic usage scenario with a language selection UI, flag icons, and multiple configuration options. The code is well-structured and follows MVVM patterns, though the core control is monolithic due to the nature of WPF custom control development. The library is published as a NuGet package and targets .NET 4.8 and .NET 6.0 (Windows).