In software engineering, especially in enterprise applications, we often face the challenge of maintaining clean architecture and ensuring separation of concerns. This becomes particularly important as applications grow larger and more complex. Two powerful concepts, MediatR and the Command Pattern, can help tackle these issues effectively by decoupling responsibilities and making code more maintainable and testable.

In this article, I’ll explain how to leverage MediatR and the Command Pattern in .NET applications to streamline communication between components and clean up your architecture. I’ll also walk through the rationale behind using these patterns and best practices for implementation.

What Is MediatR?

MediatR is a .NET library that facilitates the Mediator Pattern. It acts as a mediator between objects, eliminating direct dependencies between them. Instead of having objects directly reference one another, MediatR provides a central point of communication. This decouples the senders and receivers, promoting a cleaner and more modular design.

MediatR is particularly useful in CQRS (Command-Query Responsibility Segregation) architectures, where commands (that modify state) and queries (that return data) are separated. It allows for a straightforward, structured way to manage these operations.

Core Concepts in MediatR

1. Requests: These are the objects you send through MediatR, typically a Command (to perform an action) or a Query (to request data).

2. Handlers: These are the components responsible for processing the requests. Each request has a corresponding handler.

3. Pipeline Behaviors: These are the middlewares that can wrap around the requests and responses, providing opportunities for cross-cutting concerns like logging, validation, and authorization.

MediatR’s strength lies in its ability to simplify communication and separate concerns. Instead of scattering business logic across controllers, services, or repositories, you consolidate that logic in handlers, improving readability and testability.

Why Use MediatR?

1. Decoupling: It promotes decoupling between components, making your architecture more modular. Components don’t need to know about each other; they only need to interact with MediatR.

2. CQRS-Friendly: It fits perfectly in CQRS architectures. Commands and Queries can be implemented as separate requests with corresponding handlers.

3. Testability: Since MediatR centralizes request handling, you can easily mock the mediator and test handlers in isolation.

4. Separation of Concerns: MediatR makes it easier to adhere to Single Responsibility Principle (SRP) by separating business logic into individual handlers.

The Command Pattern

The Command Pattern is a behavioral design pattern that turns a request into a stand-alone object. This encapsulates all the information needed to perform an action, including the operation itself, parameters, and receiver. The Command Pattern decouples the object that invokes an operation from the object that knows how to perform it.

In a typical implementation, the Command is an object that represents an action. It contains all the data required for the action and is sent to an Invoker (like MediatR) to execute it. The logic for processing the command resides in the Handler, which processes the request and executes the appropriate action.

Components of the Command Pattern

1. Command: Encapsulates the request, including necessary data.

2. Invoker: A component responsible for sending the command to the correct handler. In the case of .NET, this role is played by MediatR.

3. Receiver: The entity that performs the actual work, represented by the Handler in a MediatR setup.

4. Handler: Processes the command and contains the business logic needed to execute the command.

Why Use the Command Pattern?

1. Separation of Responsibilities: The pattern allows for clear separation between the issuer of a request and the executor of the command. The issuer doesn't need to know how the command will be handled.

2. Flexibility: Commands can easily be extended or modified without affecting the rest of the system.

3. Undo Operations: Since the command encapsulates all the details of the request, it can store enough state to implement undo or redo functionality.

4. Scalability: You can add new commands without altering existing ones, making the system more extensible and maintainable.

MediatR + Command Pattern: The Perfect Combination

Combining MediatR with the Command Pattern is a powerful way to handle operations and decouple logic in your application. Here’s how they fit together:

1. Command as a Request: In MediatR, a Command becomes a request (a class implementing IRequest), encapsulating the necessary data for an operation.

2. Handler as Command Executor: The handler serves as the command executor. It receives the command, processes it, and performs the necessary business logic.

3. Invoker as MediatR: MediatR itself acts as the invoker. It routes the request to the appropriate handler.

Example Implementation

Let’s go through an example where we implement a simple "Create Order" command using MediatR and the Command Pattern.

1. Defining the Command (Request)

Here, CreateOrderCommand encapsulates the details of an order that will be created. It implements IRequest<int>, meaning the handler will return an integer (representing, for example, the order ID).

2. Creating the Command Handler

The CreateOrderHandler processes the command. It handles the logic for creating an order, typically by interacting with an underlying service or repository.

3. Invoking the Command via MediatR

In your application, you would invoke the command using MediatR like this:

The OrderController simply sends the command through MediatR. MediatR takes care of routing the command to its handler, allowing the controller to remain decoupled from the actual logic of creating an order.

Best Practices

1. Use a Request-Response Pattern: For commands, return a result such as an ID or status. This allows better interaction between the caller and the command handler.

2. Encapsulate Business Logic in Handlers: Keep the logic in the command handler and avoid bloating your controllers or other service layers.

3. Use Pipeline Behaviors: Leverage MediatR’s pipeline behaviors for cross-cutting concerns like validation, logging, or transaction management.

4. Unit Test Handlers in Isolation: Handlers should be fully unit testable since they contain the business logic. Mock MediatR in your tests to ensure the handlers work as expected.

Conclusion

MediatR, combined with the Command Pattern, can significantly improve the maintainability and scalability of your application. By embracing these patterns, you create a clean separation of concerns, improve testability, and allow your application to grow without becoming bogged down in tightly coupled components.

If you're building modern .NET applications and especially if you're using CQRS, MediatR provides a powerful, yet simple way to manage inter-component communication. The Command Pattern further strengthens this by encapsulating operations into standalone objects, providing a robust foundation for building scalable systems.