Decoupling Components in C# Applications with MediatR

In today's software development world, creating maintainable, extensible, and testable code is essential. One of the keys to achieving these goals is decoupling components, which means reducing the interdependencies between different parts of an application. In this blog post, we will explore how to decouple components in C# applications using Mediator and MediatR.

Mediator Design Pattern

The Mediator pattern is a behavioral design pattern that enables communication between different components of an application without them having direct references to one another. In this pattern, a mediator object acts as an intermediary between components, allowing them to communicate without being aware of each other's existence. The mediator encapsulates the interaction logic, making it easier to change or extend it without affecting the rest of the application.

MediatR Library

MediatR is an open-source library for .NET that implements the Mediator pattern. It provides a simple, lightweight, and extensible framework for decoupling components in C# applications. MediatR uses a set of interfaces and classes that enable communication between components through a mediator object.

MediatR implements two main types of messages: requests and notifications. A request message represents a request for data or an operation to be performed, and a notification message represents an event that has occurred. Requests and notifications are sent to a mediator object, which then routes them to the appropriate handler. Handlers are responsible for processing messages and producing a response, if necessary.

Using MediatR

To use MediatR in a C# application, we first need to install the MediatR NuGet package. Once installed, we can create request and notification messages by implementing the IRequest and INotification interfaces, respectively. We can also create handler classes that implement IRequestHandler or INotificationHandler interfaces, depending on the type of message being processed.

Let's take a look at an example of how to use MediatR to decouple components in a C# application. Suppose we have an application that allows users to create and view blog posts. We want to decouple the components responsible for creating blog posts and viewing them to make the application more maintainable and testable.

First, we create a request message for creating a blog post. We can do this by implementing the IRequest interface and defining the properties and methods necessary to create a blog post:
public class CreateBlogPostRequest : IRequest<int>
{
    public string Title { get; set; }
    public string Content { get; set; }
}
Here, we have defined a request message that includes the title and content of a blog post. The IRequest interface specifies that the message should return an integer value, which represents the ID of the created blog post.

Next, we create a handler class that will process the CreateBlogPostRequest message. We can do this by implementing the IRequestHandler interface and defining the Handle method:
public class CreateBlogPostHandler : IRequestHandler<CreateBlogPostRequest, int>
{
    private readonly IDbContext _dbContext;

    public CreateBlogPostHandler(IDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public async Task<int> Handle(CreateBlogPostRequest request, CancellationToken cancellationToken)
    {
        var blogPost = new BlogPost
        {
            Title = request.Title,
            Content = request.Content
        };

        _dbContext.BlogPosts.Add(blogPost);
        await _dbContext.SaveChangesAsync(cancellationToken);

        return blogPost.Id;
    }
}
Here, we have defined a handler class that uses an instance of IDbContext to add a new blog post to the database and return its ID. The IRequestHandler interface specifies that the handler should take a CreateBlogPostRequest message as input and return an integer value, which represents the ID of the created blog post.

Finally, we can use MediatR to send a request message to the mediator object, which will route it to the appropriate handler. We can do this by injecting an instance of IMediator into our controller or service class and calling the Send method:
public class BlogPostService : IBlogPostService
{
    private readonly IMediator _mediator;

    public BlogPostService(IMediator mediator)
    {
        _mediator = mediator;
    }

    public async Task<int> CreateBlogPost(CreateBlogPostRequest request)
    {
        return await _mediator.Send(request);
    }
}
Here, we have defined a BlogPostService class that uses an instance of IMediator to send a CreateBlogPostRequest message to the mediator object. The Send method will route the message to the appropriate handler, which in this case is the CreateBlogPostHandler class we defined earlier.

By using MediatR in this way, we have decoupled the components responsible for creating and viewing blog posts, making the application more maintainable and testable. We can easily extend the application by adding new request and notification messages and their corresponding handlers without affecting the rest of the application.

Conclusion

In this blog post, we have explored how to use Mediator and MediatR to decouple components in C# applications. MediatR provides a simple and extensible framework for implementing the Mediator pattern, allowing components to communicate without having direct references to one another. By using MediatR, we can create more maintainable, extensible, and testable applications, making it easier to develop and maintain software over time.

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