The CQRS Pattern In C#

The CQRS (Command Query Responsibility Segregation) pattern in C# is a software architectural pattern that separates the responsibilities for reading and writing data in an application. The CQRS pattern is often used in high-traffic applications, where it can help to improve performance and scalability. It can also be used to improve the security and maintainability of applications.

In CQRS, the system is divided into two main parts: the Command side and the Query side.

Command Side

  1. The Command side is responsible for handling operations that modify the state of the application (e.g., creating, updating, or deleting data).
  2. It uses commands to represent these operations, and these commands are processed by command handlers.
  3. Command handlers update the application's data store and may trigger domain logic and validation.

Query Side

  1. The Query side is responsible for reading data from the application's data store.
  2. It uses queries to retrieve data in a format suitable for presentation (e.g., UI).
  3. Query handlers are responsible for processing queries and returning the requested data.

Here's how to implement the CQRS pattern in C# with examples:

Define Commands and Queries

Create classes to represent commands and queries. Commands typically contain the data needed to perform an action, while queries specify the data to retrieve.

// Command public class CreateUserCommand { public string Username { get; set; } public string Email { get; set; } // Other properties... } // Query public class GetUserQuery { public int UserId { get; set; } }

Implement Command Handlers

Create command handlers that process commands and perform actions. These handlers can encapsulate domain logic and data manipulation.

public class UserCommandHandler { public void Handle(CreateUserCommand command) { // Create a new user with the provided data and save to the database. } }

Implement Query Handlers

Create query handlers that process queries and retrieve data from the data store.

public class UserQueryHandler { public UserDto Handle(GetUserQuery query) { // Retrieve user data from the database and return as a DTO. } }

Use a Mediator or Dispatcher

To decouple commands and queries from their handlers, you can use a mediator or dispatcher pattern. This central component routes commands and queries to their respective handlers.

public interface IMediator { TResponse Send<TResponse>(IRequest<TResponse> request); } public class Mediator : IMediator { public TResponse Send<TResponse>(IRequest<TResponse> request) { // Determine if it's a command or query and route to the appropriate handler. // Example: Handle commands using UserCommandHandler and queries using UserQueryHandler. } }

In your application, you can create and send commands and queries using the mediator:

var mediator = new Mediator(); var createUserCommand = new CreateUserCommand { Username = "user1", Email = "" }; mediator.Send(createUserCommand); var getUserQuery = new GetUserQuery { UserId = 1 }; var userDto = mediator.Send(getUserQuery);

Benefits of using the CQRS pattern

The CQRS pattern can be a bit more complex to implement than a traditional approach, but it can offer a number of benefits, such as improved performance, scalability, security, and maintainability.

  1. Improved performance: The CQRS pattern can help to improve performance by separating the read and write operations. This can be especially beneficial in high-traffic applications.
  2. Improved scalability: The CQRS pattern can help to improve scalability by making it easier to scale the read and write operations independently.
  3. Improved security: The CQRS pattern can help to improve security by isolating the read and write operations. This can make it more difficult for attackers to gain access to sensitive data.
  4. Improved maintainability: The CQRS pattern can help to improve maintainability by making the code more modular and reusable.
Example: CQRS pattern implementation in C#

Here is an example of a simple CQRS pattern implementation in C#:

// Command model public class CreateCustomerCommand { public string Name { get; set; } public string Email { get; set; } } // Query model public class CustomerDto { public int Id { get; set; } public string Name { get; set; } public string Email { get; set; } } // Command handler public class CreateCustomerCommandHandler { private readonly ICustomerRepository customerRepository; public CreateCustomerCommandHandler(ICustomerRepository customerRepository) { this.customerRepository = customerRepository; } public void Handle(CreateCustomerCommand command) { var customer = new Customer { Name = command.Name, Email = command.Email }; customerRepository.Add(customer); } } // Query handler public class GetCustomerByIdQueryHandler { private readonly ICustomerRepository customerRepository; public GetCustomerByIdQueryHandler(ICustomerRepository customerRepository) { this.customerRepository = customerRepository; } public CustomerDto Handle(GetCustomerByIdQuery query) { var customer = customerRepository.GetById(query.Id); return new CustomerDto { Id = customer.Id, Name = customer.Name, Email = customer.Email }; } }

In this example, the CreateCustomerCommand and GetCustomerByIdQuery classes represent the command and query models, respectively. The CreateCustomerCommandHandler and GetCustomerByIdQueryHandler classes represent the command and query handlers, respectively.

To use the CQRS pattern, you would simply send commands to the command handlers and queries to the query handlers.

// Create a new customer var command = new CreateCustomerCommand { Name = "Miller Joe", Email = "" }; commandHandler.Handle(command); // Get the customer by ID var query = new GetCustomerByIdQuery { Id = 1 }; var customerDto = queryHandler.Handle(query);


The CQRS (Command Query Responsibility Segregation) pattern in C# involves separating the responsibilities for handling commands that modify application state from queries that retrieve data. It is implemented by defining command and query classes, creating corresponding command and query handlers, and using a mediator or dispatcher to route them to their respective handlers, enabling improved scalability and maintainability in complex applications.