DelegatingHandlers

DelegatingHandlers are a key component in the .NET Web API framework, specifically designed to enable developers to add custom logic to HTTP request and response pipelines. Introduced in ASP.NET Web API 2, they allow for the interception and modification of HTTP messages before they reach the route handlers or after they are processed by the controller actions. This mechanism is particularly useful for implementing cross-cutting concerns such as authentication, logging, caching, or request validation without cluttering controller code.

A DelegatingHandler is essentially a class that inherits from the abstract `DelegatingHandler` class in the `System.Net.Http` namespace. It implements the `SendAsync` method, which receives an `HttpRequestMessage` and returns an `HttpResponseMessage`. The handler can inspect, modify, or even block the request or response before passing it to the next handler in the pipeline. This chaining of handlers allows for a modular and extensible approach to handling HTTP requests and responses.

To use a DelegatingHandler, it must be registered with the HTTP client or the OWIN pipeline. In ASP.NET Core, handlers are typically added to the `HttpClient` or configured in the middleware pipeline. For example, a custom handler can be added to an `HttpClient` instance like this:

var client = new HttpClient();

client.DefaultRequestHeaders.Add("Authorization", "Bearer token");

client.DefaultRequestHeaders.Add("CustomHeader", "Value");

client.Handlers.Add(new CustomDelegatingHandler());

Alternatively, in ASP.NET Core middleware, handlers can be configured in the `ConfigureServices` method to intercept requests globally. This approach is useful for centralized logic like logging or authentication that applies across the entire application.

DelegatingHandlers are also widely used in client-side applications to add custom behavior to HTTP requests, such as retry logic, request timeouts, or custom headers. They provide a clean separation of concerns and promote reusability, making them a powerful tool for building scalable and maintainable web applications.