Adding Behavior Without Modifying Code
The decorator pattern allows you to add behavior to objects dynamically without modifying their implementation. This is particularly useful for cross-cutting concerns like logging, caching, validation, and retry logic. Decorator pattern in C# with Needlr simplifies decorator implementation by automatically discovering and wiring decorators, eliminating the manual registration boilerplate that typically accompanies this pattern.
Needlr provides multiple ways to implement decorators: the [DecoratorFor] attribute for automatic discovery, manual decorator registration through plugins, and integration with Scrutor for advanced decoration scenarios. This article explains how to use each approach, when to choose one over another, and how decorators integrate with Needlr's automatic service discovery.
If you are new to the decorator pattern, the guide on how to implement the decorator pattern with Autofac provides foundational concepts. For developers familiar with decorators, this article shows how Needlr simplifies the implementation while maintaining flexibility.
This article focuses specifically on decorator implementation with Needlr. Topics like automatic service discovery, plugin architecture, and service lifetimes are covered in other articles. Here we are concerned with adding cross-cutting concerns through decorators.
Understanding the Decorator Pattern
The decorator pattern involves wrapping an object with another object that implements the same interface, delegating calls to the wrapped object while adding additional behavior. This allows you to compose behavior dynamically without modifying the original implementation.
Common Decorator Use Cases
- Logging: Adding method entry/exit logging without modifying service implementations
- Caching: Wrapping services with caching logic to improve performance
- Validation: Adding input validation before service methods execute
- Retry Logic: Implementing automatic retry for transient failures
- Performance Monitoring: Tracking execution time and performance metrics
- Security: Adding authorization checks before method execution
Here is a basic example without Needlr:
public interface IOrderService
{
Task<Order> CreateOrderAsync(OrderRequest request);
}
// Original service
public class OrderService : IOrderService
{
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
// Business logic
return await Task.FromResult(new Order());
}
}
// Decorator that adds logging
public class LoggingOrderService : IOrderService
{
private readonly IOrderService _inner;
private readonly ILogger<LoggingOrderService> _logger;
public LoggingOrderService(IOrderService inner, ILogger<LoggingOrderService> logger)
{
_inner = inner;
_logger = logger;
}
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
_logger.LogInformation("Creating order for customer {CustomerId}", request.CustomerId);
var order = await _inner.CreateOrderAsync(request);
_logger.LogInformation("Order {OrderId} created", order.Id);
return order;
}
}
// Manual registration (without Needlr)
services.AddSingleton<OrderService>();
services.AddSingleton<IOrderService>(sp =>
new LoggingOrderService(
sp.GetRequiredService<OrderService>(),
sp.GetRequiredService<ILogger<LoggingOrderService>>()));
This manual approach works but requires careful registration ordering and becomes complex when you have multiple decorators. Needlr simplifies this significantly.
Automatic Decorator Discovery with [DecoratorFor]
Needlr's [DecoratorFor] attribute is the simplest way to implement decorators. You mark a decorator class with the attribute, and Needlr automatically discovers and wires it up.
Here is the same example using [DecoratorFor]:
using NexusLabs.Needlr;
using NexusLabs.Needlr.Injection;
using NexusLabs.Needlr.Injection.SourceGen;
// Original service (automatically discovered)
public interface IOrderService
{
Task<Order> CreateOrderAsync(OrderRequest request);
}
public class OrderService : IOrderService
{
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
// Business logic
return await Task.FromResult(new Order());
}
}
// Decorator with automatic discovery
[DecoratorFor(typeof(IOrderService))]
public class LoggingOrderService : IOrderService
{
private readonly IOrderService _inner;
private readonly ILogger<LoggingOrderService> _logger;
public LoggingOrderService(IOrderService inner, ILogger<LoggingOrderService> logger)
{
_inner = inner;
_logger = logger;
}
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
_logger.LogInformation("Creating order for customer {CustomerId}", request.CustomerId);
var order = await _inner.CreateOrderAsync(request);
_logger.LogInformation("Order {OrderId} created", order.Id);
return order;
}
}
// Setup - decorators are automatically discovered and wired
var serviceProvider = new Syringe()
.UsingSourceGen()
.BuildServiceProvider();
var orderService = serviceProvider.GetRequiredService<IOrderService>();
// orderService is actually LoggingOrderService wrapping OrderService
The [DecoratorFor(typeof(IOrderService))] attribute tells Needlr that LoggingOrderService decorates IOrderService. Needlr automatically:
- Discovers
OrderServiceand registers it - Discovers
LoggingOrderServiceand recognizes it as a decorator - Wires them together so that
LoggingOrderServicewrapsOrderService
You do not need to write any registration code. The decorator is automatically applied.
Multiple Decorators with Ordering
When you have multiple decorators for the same service, you can control the order using the Order property of the [DecoratorFor] attribute. Lower order values are applied first (closer to the original service), and higher order values wrap outer layers.
Here is an example with multiple decorators:
using NexusLabs.Needlr;
public interface IOrderService
{
Task<Order> CreateOrderAsync(OrderRequest request);
}
public class OrderService : IOrderService
{
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
return await Task.FromResult(new Order());
}
}
// Applied first (closest to OrderService)
[DecoratorFor(typeof(IOrderService), Order = 1)]
public class LoggingOrderService : IOrderService
{
private readonly IOrderService _inner;
private readonly ILogger<LoggingOrderService> _logger;
public LoggingOrderService(IOrderService inner, ILogger<LoggingOrderService> logger)
{
_inner = inner;
_logger = logger;
}
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
_logger.LogInformation("Creating order");
var order = await _inner.CreateOrderAsync(request);
_logger.LogInformation("Order created");
return order;
}
}
// Applied second (wraps LoggingOrderService)
[DecoratorFor(typeof(IOrderService), Order = 2)]
public class CachingOrderService : IOrderService
{
private readonly IOrderService _inner;
private readonly ICache _cache;
public CachingOrderService(IOrderService inner, ICache cache)
{
_inner = inner;
_cache = cache;
}
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
var cacheKey = $"order:{request.CustomerId}";
if (_cache.TryGetValue(cacheKey, out Order cachedOrder))
{
return cachedOrder;
}
var order = await _inner.CreateOrderAsync(request);
_cache.Set(cacheKey, order);
return order;
}
}
When IOrderService is resolved, the decorator chain is:
CachingOrderService → LoggingOrderService → OrderService
The caching decorator wraps the logging decorator, which wraps the original service. This ordering ensures that caching happens at the outermost layer, while logging happens for every call, including cache hits.
Manual Decorator Registration
Sometimes you need more control over decorator registration than the [DecoratorFor] attribute provides. Perhaps you need conditional decoration, factory-based creation, or integration with existing manual registrations. In these cases, you can register decorators manually.
Here is an example of manual decorator registration:
using Microsoft.Extensions.DependencyInjection;
using NexusLabs.Needlr;
// Mark the original service to prevent auto-registration
[DoNotAutoRegister]
public class OrderService : IOrderService
{
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
return await Task.FromResult(new Order());
}
}
// Mark the decorator to prevent auto-registration
[DoNotAutoRegister]
public class LoggingOrderService : IOrderService
{
private readonly IOrderService _inner;
private readonly ILogger<LoggingOrderService> _logger;
public LoggingOrderService(IOrderService inner, ILogger<LoggingOrderService> logger)
{
_inner = inner;
_logger = logger;
}
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
_logger.LogInformation("Creating order");
return await _inner.CreateOrderAsync(request);
}
}
// Plugin that manually registers the decorator
internal sealed class OrderServicePlugin : IServiceCollectionPlugin
{
public void Configure(ServiceCollectionPluginOptions options)
{
// Register the original service
options.Services.AddSingleton<OrderService>();
// Manually wire the decorator
options.Services.AddSingleton<IOrderService>(sp =>
{
var inner = sp.GetRequiredService<OrderService>();
var logger = sp.GetRequiredService<ILogger<LoggingOrderService>>();
return new LoggingOrderService(inner, logger);
});
}
}
Manual registration gives you complete control over how decorators are wired, but it requires more code. Use this approach when you need conditional logic or factory-based creation that the [DecoratorFor] attribute cannot express.
Using AddDecorator Extension
Needlr provides an AddDecorator extension method that simplifies manual decorator registration:
using NexusLabs.Needlr.Injection;
using NexusLabs.Needlr.Injection.SourceGen;
var serviceProvider = new Syringe()
.UsingSourceGen()
.UsingPostPluginRegistrationCallback(services =>
{
// Register the base service
services.AddSingleton<OrderService>();
})
.AddDecorator<IOrderService, LoggingOrderService>()
.BuildServiceProvider();
The AddDecorator extension automatically wraps the existing service registration with the decorator, preserving the original service's lifetime. This is cleaner than manual factory-based registration while still giving you explicit control.
Real-World Example: E-Commerce Service with Multiple Decorators
Let's look at a complete example that demonstrates multiple decorators working together:
using NexusLabs.Needlr;
using NexusLabs.Needlr.Injection;
using NexusLabs.Needlr.Injection.SourceGen;
var serviceProvider = new Syringe()
.UsingSourceGen()
.BuildServiceProvider();
var orderService = serviceProvider.GetRequiredService<IOrderService>();
await orderService.CreateOrderAsync(new OrderRequest());
// Original service
public interface IOrderService
{
Task<Order> CreateOrderAsync(OrderRequest request);
}
public class OrderService : IOrderService
{
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
// Business logic
await Task.Delay(100); // Simulate work
return new Order { Id = Guid.NewGuid() };
}
}
// Validation decorator (applied first)
[DecoratorFor(typeof(IOrderService), Order = 1)]
public class ValidatingOrderService : IOrderService
{
private readonly IOrderService _inner;
public ValidatingOrderService(IOrderService inner)
{
_inner = inner;
}
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
if (request == null)
{
throw new ArgumentNullException(nameof(request));
}
if (request.CustomerId <= 0)
{
throw new ArgumentException("Invalid customer ID", nameof(request));
}
return await _inner.CreateOrderAsync(request);
}
}
// Logging decorator (applied second)
[DecoratorFor(typeof(IOrderService), Order = 2)]
public class LoggingOrderService : IOrderService
{
private readonly IOrderService _inner;
private readonly ILogger<LoggingOrderService> _logger;
public LoggingOrderService(IOrderService inner, ILogger<LoggingOrderService> logger)
{
_inner = inner;
_logger = logger;
}
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
_logger.LogInformation("Creating order for customer {CustomerId}", request.CustomerId);
try
{
var order = await _inner.CreateOrderAsync(request);
_logger.LogInformation("Order {OrderId} created successfully", order.Id);
return order;
}
catch (Exception ex)
{
_logger.LogError(ex, "Failed to create order for customer {CustomerId}", request.CustomerId);
throw;
}
}
}
// Retry decorator (applied third, outermost)
[DecoratorFor(typeof(IOrderService), Order = 3)]
public class RetryOrderService : IOrderService
{
private readonly IOrderService _inner;
private readonly int _maxRetries;
public RetryOrderService(IOrderService inner)
{
_inner = inner;
_maxRetries = 3;
}
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
for (int attempt = 1; attempt <= _maxRetries; attempt++)
{
try
{
return await _inner.CreateOrderAsync(request);
}
catch (Exception) when (attempt < _maxRetries)
{
await Task.Delay(100 * attempt); // Exponential backoff
}
}
throw new InvalidOperationException("Failed after retries");
}
}
When IOrderService is resolved, the decorator chain is:
RetryOrderService → LoggingOrderService → ValidatingOrderService → OrderService
This ordering ensures that:
- Validation happens first (closest to the original service)
- Logging captures all calls, including retries
- Retry logic wraps everything, handling transient failures
Decorating Multiple Services
A single decorator class can decorate multiple services by using multiple [DecoratorFor] attributes:
using NexusLabs.Needlr;
// Decorator that works for multiple services
[DecoratorFor(typeof(IOrderService))]
[DecoratorFor(typeof(IPaymentService))]
[DecoratorFor(typeof(IShippingService))]
public class LoggingServiceDecorator : IOrderService, IPaymentService, IShippingService
{
private readonly object _inner;
private readonly ILogger _logger;
public LoggingServiceDecorator(object inner, ILogger<LoggingServiceDecorator> logger)
{
_inner = inner;
_logger = logger;
}
// Implement all interfaces, delegating to inner
public async Task<Order> CreateOrderAsync(OrderRequest request)
{
_logger.LogInformation("Creating order");
return await ((IOrderService)_inner).CreateOrderAsync(request);
}
public async Task<PaymentResult> ProcessPaymentAsync(PaymentRequest request)
{
_logger.LogInformation("Processing payment");
return await ((IPaymentService)_inner).ProcessPaymentAsync(request);
}
public async Task<ShippingLabel> CreateShippingLabelAsync(Order order)
{
_logger.LogInformation("Creating shipping label");
return await ((IShippingService)_inner).CreateShippingLabelAsync(order);
}
}
This pattern is useful when you have cross-cutting concerns that apply to multiple services and you want a single decorator implementation to handle them all.
Integration with Scrutor
If you are using Needlr with Scrutor's type registrar, you can leverage Scrutor's decoration extensions for even more advanced scenarios:
using NexusLabs.Needlr.Injection;
using NexusLabs.Needlr.Injection.Reflection;
using NexusLabs.Needlr.Injection.Scrutor;
using Microsoft.Extensions.DependencyInjection;
var serviceProvider = new Syringe()
.UsingReflection()
.UsingScrutorTypeRegistrar()
.UsingPostPluginRegistrationCallback(services =>
{
// Register base service
services.AddSingleton<OrderService>();
// Use Scrutor to decorate
services.Decorate<IOrderService, LoggingOrderService>();
services.Decorate<IOrderService, CachingOrderService>();
})
.BuildServiceProvider();
Scrutor's Decorate extension provides additional features like conditional decoration and more complex decoration scenarios. This is useful when you need capabilities beyond what [DecoratorFor] provides.
Best Practices
When implementing decorators with Needlr, follow these best practices:
Use [DecoratorFor] by Default: Start with the [DecoratorFor] attribute for automatic discovery. It is the simplest approach and covers most use cases.
Order Decorators Carefully: Use the Order property to control decorator ordering. Lower values are closer to the original service, higher values wrap outer layers.
Keep Decorators Focused: Each decorator should add a single cross-cutting concern. This makes decorators easier to understand, test, and compose.
Avoid Business Logic in Decorators: Decorators should handle infrastructure concerns like logging, caching, and retries. Business logic belongs in the original service.
Test Decorators Independently: Decorators can be tested independently by creating mock inner services. This makes testing easier and more focused.
Comparison with Other DI Containers
Needlr's decorator support compares favorably with other dependency injection containers:
Autofac: Autofac requires explicit decorator registration with RegisterDecorator. Needlr's [DecoratorFor] attribute eliminates this boilerplate.
Simple Injector: Simple Injector uses attributes but requires explicit registration. Needlr combines attributes with automatic discovery.
Built-in Container: The built-in Microsoft container does not support decorators natively. You need libraries like Scrutor or manual factory registration.
Needlr's approach provides the convenience of automatic discovery with the flexibility of manual registration when needed.
Conclusion
The decorator pattern is a powerful way to add cross-cutting concerns to your services without modifying their implementation. Needlr simplifies decorator implementation through automatic discovery with the [DecoratorFor] attribute, while still supporting manual registration for complex scenarios.
Whether you need simple logging decorators or complex chains of multiple decorators, Needlr provides the tools to implement them cleanly and maintainably. The combination of automatic discovery and flexible manual registration gives you the best of both worlds: convention-based defaults with the ability to customize when needed.
Frequently Asked Questions
How does Needlr discover decorators?
Needlr discovers decorators automatically during assembly scanning, just like regular services. Classes marked with [DecoratorFor] are recognized as decorators and automatically wired to the services they decorate.
Can I have multiple decorators for the same service?
Yes, you can have multiple decorators for the same service. Use the Order property of [DecoratorFor] to control the order in which decorators are applied.
How do I control decorator ordering?
Use the Order property of the [DecoratorFor] attribute. Lower order values are applied first (closer to the original service), and higher values wrap outer layers.
Can I use decorators with manually registered services?
Yes, you can use the AddDecorator extension method or manual factory registration to decorate manually registered services. Mark services with [DoNotAutoRegister] to prevent automatic discovery if needed.
Does Needlr support conditional decoration?
Yes, you can use manual registration with conditional logic to apply decorators conditionally. The [DecoratorFor] attribute always applies the decorator, so use manual registration for conditional scenarios.
Can a single decorator decorate multiple services?
Yes, you can use multiple [DecoratorFor] attributes on a single class to decorate multiple services. The decorator class must implement all the interfaces it decorates.
How do decorators work with service lifetimes?
Decorators inherit the lifetime of the service they decorate. If the original service is registered as a singleton, the decorator chain is also a singleton. If it is transient, the entire chain is transient.