SOLID & Design Patterns Interview Question #96

How can the Decorator Pattern be used for logging or validation?

DDD Boundaries, Resilience, Caching & Practical ASP.NET Core Design Mid-Level Intermediate

Quick Interview Answer

A decorator can execute cross-cutting behavior before or after delegating to the wrapped service. For example, a logging decorator can measure execution time, while a validation decorator can reject invalid input before the core service runs.

Detailed Explanation

This avoids duplicating the same concern across multiple implementations. Cross-cutting behavior should still be placed at the most appropriate boundary; not every concern needs a decorator.

Code Example

public class ValidatingOrderService : IOrderService
{
    private readonly IOrderService _inner;

    public Task PlaceAsync(OrderRequest request)
    {
        if (request.Items.Count == 0)
            throw new ValidationException("Items required.");

        return _inner.PlaceAsync(request);
    }
}