DDD: Business Logic & Exceptions in Entities

One of the key components of DDD is the concept of entities, which encapsulate both data and behavior within the domain. In this blog post, we will explore how to handle business logic and exceptions within entities using C# as our programming language.

Entities and Business Logic

In DDD, entities represent the core objects within a domain. They encapsulate both state and behavior, allowing us to model complex business rules and operations. Business logic, such as validations, calculations, or state transitions, can be implemented within entities to ensure the integrity and consistency of the domain.

Let's take an example of an Order entity in an e-commerce system. The Order entity may have various business rules associated with it, such as ensuring that the order has a valid customer, contains at least one item, or meets certain criteria for processing.

public class Order
{
    public int Id { get; set; }
    public Customer Customer { get; set; }
    public List<OrderItem> Items { get; set; }

    public void AddItem(OrderItem item)
    {
        if (item == null)
        {
            throw new ArgumentNullException(nameof(item));
        }

        // Additional business logic for item validation

        Items.Add(item);
    }

    // Other methods and properties...
}

In the code snippet above, we define an Order entity with properties like Id, Customer, and Items. The AddItem method demonstrates how business logic can be incorporated into the entity. Here, we validate the item parameter and perform any additional business-specific checks before adding it to the Items collection.

By keeping the business logic within the entity, we ensure that the domain rules are enforced consistently whenever an operation is performed on an Order object.

Handling Exceptions

Exceptions play a crucial role in error handling and notifying clients about exceptional scenarios. In DDD, exceptions can be used to communicate domain-specific errors or violations of business rules.

Continuing with our Order entity example, let's consider a scenario where the order must have a minimum total value for processing. If the order fails to meet this criteria, we can throw a custom exception to indicate the violation.

public class InsufficientOrderValueException : Exception
{
    public InsufficientOrderValueException(string message) : base(message)
    {
    }
}

public class Order
{
    // Properties and methods...

    public void Process()
    {
        decimal totalValue = CalculateTotalValue();

        if (totalValue < minimumValue)
        {
            throw new InsufficientOrderValueException("The order does not meet the minimum value requirement.");
        }

        // Proceed with order processing
    }

    // Other methods and properties...
}

In the code snippet above, we define a custom exception InsufficientOrderValueException to represent the specific violation. Within the Process method, we calculate the total value of the order and check if it meets the minimum requirement. If not, we throw the custom exception with an appropriate error message.

By using custom exceptions, we can differentiate between different types of errors and handle them accordingly in our application or propagate them to the client for appropriate actions.