CQRS Pattern With C# and MediatR (Part 2)

Implement CQRS pattern with MediatR library in a real world application

Abdelmajid BACO
3 min readAug 7, 2020
Implementing CQRS pattern with MediatR library
Image from Pexels, a pick guitar or Médiator in french

This article is the continuation of the first one already published about “CQRS pattern with C# (Part 1)”, therefore, we will not go to explain CQRS proposal again. In this second part we are going to speak about MediatR Library and how it can implement read and write operations defined by CQRS.

MediatR Project Logo

By the end of this article we should understand how to use the MediateR Library to achieve the following goals:

1- Implement Commands and Queries operations.

2- Implement Notifications.

3- Use Dependency Injection to create a new instance of MediatR.

4- Use MediatR with Console application and Web API application.

MediatR Library

According to MediatR GitHub repository:

MediatR is a low-ambition library trying to solve a simple problem — decoupling the in-process sending of messages from handling messages. Cross-platform, supporting .NET Framework 4.6.1 and netstandard2.0.

In simple word, MediatR Library allows to dispatch messages to the appropriate handlers. It manages two kinds of messages:

- Requests/Responses, are commands and queries operations. Each operation has its own handler and may or not return a value.

- Notifications, are events raised due to the system modifications. Each event can have several handlers.

The following schema gives an overview of the MediatR Library purpose.

CQRS Architecture in Domain-Driven Design (DDD) and MediatR libradry
MediatR Library purpose

Install MediatR Library

To install the package via Nuget type the command: Install-Package MediatR or use .NET Core command line like this: dotnet add package MediatR

Real World Application

To represent a request (queries and commands) with response, MediatR Library uses a marker interface named IRequest<out TResponse>. For example:

“Add a New Product” : this scenario is a command operation because it’ll make a change to the system.

I created a class called AddNewProductCommand, this class is a simple DTO object that implement IRequest.

Command class example

Each command/query has its own handler, AddNewProductCommandHandler defines a handler for AddNewProductCommand.

Add a new product handler

To raise an event after a system modification we use the following code: await _mediator.Publish(new ProductCreated(product)).

This instruction will be performed by the appropriate notification handler, in this case the class NewProductCreatedHandler.

Product created notification handler

The ProductCreated class must implement the marker interface to represent a notification. Like this:

Product created notification

Register MediatR

Now, if you have a webApi project you can register MediatR in Startup.cs using ConfigurationdServices method. Make sure you have installed Microsoft.Extensions.DependencyInjection before.

ConfigureServices method

If you have a console project you can use the following method to register MediatR Library.

Build Mediator with console Application

The following code shows the scenario “Add a new Product” API Controller.

Add new Product with MediatR

You can get all the source code from GitHub:

Thanks for reading. I hope this article was helpful.

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Abdelmajid BACO

Senior Full Stack .Net / Angular Developer, Cloud & Azure DevOps, Carrier Manager, Husband, and Father.