Custom Attributes, `AttributeUsage` & Runtime Metadata Extraction

1Concept

Custom attributes inherit from `System.Attribute` and embed serialized metadata blobs into assembly tables. `AttributeUsage` restricts target targets (Class, Method, Property) and controls inheritance.

2Architecture Diagram

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class EndpointRouteAttribute : Attribute
       │
       ▼
Decorates Class / Method ──> Extracted via `member.GetCustomAttribute<T>()` 

3Code Example

using System;
using System.Reflection;

[AttributeUsage(AttributeTargets.Method, Inherited = false, AllowMultiple = false)]
public sealed class HttpPostAttribute(string route) : Attribute
{
    public string Route { get; } = route;
}

public class OrderController
{
    [HttpPost("/api/orders/checkout")]
    public void Checkout() => Console.WriteLine("Processing checkout...");
}

public class CustomAttributeDemo
{
    public static void Main()
    {
        var method = typeof(OrderController).GetMethod("Checkout");
        var attr = method?.GetCustomAttribute<HttpPostAttribute>();

        Console.WriteLine($"Discovered Endpoint Route: {attr?.Route}");
    }
}

4Expected Output

Discovered Endpoint Route: /api/orders/checkout

5Key Takeaways

• ✓Always seal custom attributes to enable compiler optimizations.
• ✓Set `Inherited = false` when attribute behavior should not cascade to derived subclasses.
• ✓Use attributes for declarative framework design (validation, routing, authorization).