Chris Straw

Domain-Driven Design (DDD) using C# from ChatGPT


I asked ChatGPT4 what a DDD is in c#. It came back with a decent answer.

In Domain-Driven Design (DDD) using C#, the models, also known as domain entities, typically reside in a separate project or folder named “Domain” or “Core”. This project or folder is usually a class library, and it should not have any dependencies on other parts of the system. This separation allows the domain models to be easily used across different layers of the application.

Here’s an example of a typical solution structure for a DDD-based C# application:

In this example, the “MyApplication.Domain” (or “MyApplication.Core”) project would contain the domain models, such as entities, value objects, aggregates, and repository interfaces, as well as domain events and services if needed.

The domain models should be kept clean from any infrastructure or application-layer concerns. They should only contain the business logic and rules that pertain to the domain itself.

What’s an example of DomainEvents

Domain events represent something that has happened within the domain that is of interest to other parts of the system. They are used to communicate between different components and are especially useful in a decoupled architecture. Domain events help you separate concerns by allowing different parts of your application to react to changes in the domain without needing a direct reference or tight coupling.

Here’s an example of a domain event in C#:

public class OrderShippedEvent : IDomainEvent
    public OrderShippedEvent(Order order, DateTime occurredAt)
        Order = order;
        OccurredAt = occurredAt;

    public Order Order { get; }
    public DateTime OccurredAt { get; }

In this example, OrderShippedEvent is a domain event representing that an order has been shipped. It contains information about the shipped order and when the event occurred. Notice that the event implements an IDomainEvent interface, which is a marker interface to distinguish domain events from other classes.

public interface IDomainEvent

When the event is raised (i.e., when an order is marked as shipped), other parts of the system can subscribe to this event and react accordingly. For example, an email notification service can listen for OrderShippedEvent and send an email to the customer when the event is raised.

To raise and handle domain events, you might use an event dispatcher or a mediator library like MediatR, which can help you manage the dispatching of domain events to their respective handlers in a decoupled manner.

What is the Messaging

Messaging, in the context of software architecture and particularly in Domain-Driven Design (DDD), refers to the process of asynchronously exchanging information between different components or systems using messages. Messages are self-contained pieces of data or information, often wrapped in a specific format like JSON or XML, that can be sent and received by different components or systems.

In a DDD-based application, messaging is often used to:

1. Decouple components or systems, allowing them to evolve independently.
2. Improve application performance by offloading time-consuming tasks to be processed asynchronously.
3. Enable communication between different microservices or bounded contexts in a distributed system.

There are different types of messaging patterns, including:

1. Point-to-Point: A message is sent from a single producer to a single consumer.
2. Publish-Subscribe: A message is sent from a producer (publisher) to multiple consumers (subscribers).
3. Request-Reply: A message is sent from a requester to a replier, and the replier sends back a response message.

To implement messaging in a C# application, you can use various libraries, frameworks, or services, such as:

1. Message queues (e.g., RabbitMQ, Apache Kafka, or Amazon SQS): These technologies provide reliable and scalable messaging infrastructure that can handle high volumes of messages and ensure they are delivered to the appropriate consumers.
2. Mediator pattern libraries (e.g., MediatR): These libraries can help manage in-process messaging by abstracting the process of dispatching messages to their respective handlers within your application.
3. SignalR: This is a library for ASP.NET Core applications that simplifies real-time web functionality, allowing server-side code to push content to connected clients instantly, using WebSockets or other fallback transports.

In the context of a DDD-based C# application, messaging-related components typically reside in the “Infrastructure” project or folder, where you would handle the integration with messaging infrastructure, serialization and deserialization of messages, and other messaging-related concerns.

What are valueobjects

In Domain-Driven Design (DDD), Value Objects are small, immutable objects that represent a concept or a value in the domain. Unlike entities, which have a unique identity and may change over time, value objects are identified solely by the combination of their properties. Two value objects with the same properties are considered equal, even if they are different instances.

Value objects should be immutable, meaning their state cannot be changed after they are created. Any operation that would alter the state of a value object should return a new instance with the desired changes, leaving the original object unchanged.

Here’s an example of a value object in C#:

public class Money : IEquatable<Money>
    public decimal Amount { get; }
    public string Currency { get; }

    public Money(decimal amount, string currency)
        Amount = amount;
        Currency = currency;

    // Implement equality based on properties, not object references
    public bool Equals(Money other)
        if (other == null) return false;
        return Amount == other.Amount && Currency == other.Currency;

    public override bool Equals(object obj)
        if (obj == null || GetType() != obj.GetType()) return false;
        return Equals(obj as Money);

    public override int GetHashCode()
        return (Amount, Currency).GetHashCode();

    // Implement any domain-specific operations, ensuring immutability
    public Money Add(Money other)
        if (Currency != other.Currency)
            throw new InvalidOperationException("Cannot add money with different currencies.");

        return new Money(Amount + other.Amount, Currency);

In this example, Money is a value object representing an amount of money in a specific currency. It is immutable and equality is based on the Amount and Currency properties. The Add method returns a new instance of Money with the added amounts, without modifying the original objects.

Value objects are useful for encapsulating domain logic and constraints, ensuring that they are consistently enforced across your application. They help make your code more expressive, maintainable, and less prone to errors by reducing the need for primitive data types and ad-hoc validation logic.

Written by

Chris Straw