MediatR

MediatR is a lightweight in-process messaging library for the .NET platform that implements the mediator pattern. It enables decoupling between components by routing requests and notifications through a centralized mediator rather than having components call each other directly. This can simplify dependencies and improve testability in applications of various sizes.

Core concepts in MediatR include IMediator, which exposes methods for sending requests and publishing notifications; IRequest<T> and IRequest for defining request messages with or without a typed response; INotification and INotificationHandler for event-style messages; and IRequestHandler and INotificationHandler to implement the logic that handles those messages. The library is designed to be registered with a dependency injection container, so handlers are discovered and resolved at runtime.

MediatR supports asynchronous operation and can be extended with pipeline behaviors. Pipeline behaviors are extensible components that wrap the invocation of a handler, enabling cross-cutting concerns such as logging, validation, performance monitoring, or authorization to run before or after the main handler logic.

Common use cases include implementing the CQRS pattern, separating commands and queries, and decoupling senders from handlers to improve maintainability and testability. It is important to note that MediatR is an in-process messaging tool and does not provide cross-process or distributed messaging capabilities.

Originating with Jimmy Bogard, MediatR is an open-source library commonly used in .NET applications and distributed via NuGet. It integrates well with ASP.NET Core and other .NET runtimes, offering a flexible, lightweight approach to in-process messaging and handler composition.