DelegatingHandler

As explained in ASP.NET Core in Action, 3e

DelegatingHandler is a .NET HttpMessageHandler base class that forwards processing to an inner handler while allowing additional behavior.

DelegatingHandler is a HttpMessageHandler base class for adding behavior around outgoing HTTP requests. It forwards processing to the next handler and can inspect or modify responses. section 33.4 section 33.5

Create and attach a custom handler in these steps:

• Derive a handler from DelegatingHandler and override SendAsync(). section 33.4
• Add the custom behavior, call base.SendAsync(), and return or modify the response. section 33.4
• Register the handler with dependency injection and attach it using AddHttpMessageHandler<T>() on a named or typed client. section 33.4
• Choose transient or singleton registration according to whether the handler requires state, and use registration order to control pipeline order. section 33.4

Adding an API key

An ApiKeyMessageHandler can inject API-key settings, add an API-KEY header to every outgoing request, and call the next handler. Register it as a transient service, then add it to an ExchangeRatesClient alongside a transient error handler. section 33.4

Figure 33.6 You can use a custom HttpMessageHandler to modify requests before they’re sent to third-party APIs. Every request passes through the custom handler before the final handler (the HttpClientHandler) sends the request to the HTTP API. After the response is received, each handler gets a chance to inspect and modify the response.

Follow the request from a custom handler through the pipeline to HttpClientHandler, then follow the response back through handlers. section 33.4

How ASP.NET Core in Action, 3e covers it

How deep it goes

Explains derivation, SendAsync, dependency-injection registration, pipeline ordering, and request-response flow.

Examples

Demonstrates an API-key handler and its registration alongside transient error handling for ExchangeRatesClient.

What it assumes

Assumes familiarity with C# classes, asynchronous methods, dependency injection, and named or typed HttpClient clients.

What you can do after

Build custom request behavior, register it with dependency injection, and apply it to selected HttpClient pipelines.

How it connects

Connects DelegatingHandler with HttpMessageHandler, HttpClientHandler, IHttpClientFactory, dependency injection, and typed or named clients.

liveBookmanning.com

Read ASP.NET Core in Action, 3e on liveBook Get the book at manning.com

Also explained in

• 

  liveBookmanning.com
  .NET MAUI in ActionA delegating handler is an HTTP message handler that can inspect or modify a request or response before passing it to another handler.

Compare how 2 books explain DelegatingHandler →

Related concepts

• AddHttpMessageHandlerused by · AddHttpMessageHandler is an IHttpClientBuilder extension method that adds a delegating handler type to an HttpClient handler pipeline.
• custom HttpMessageHandlerincludes · A custom HttpMessageHandler is an application-defined HTTP pipeline component that modifies or observes requests and responses.
• HttpMessageHandlerkind of · HttpMessageHandler is an abstract .NET component that processes HTTP request and response messages within an HttpClient handler chain.
• HTTP clientuses · An HTTP client is software or an API component that sends HTTP requests and receives and processes HTTP responses.
• single responsibility principleimplements · The single responsibility principle is a software design principle that gives a component one focused responsibility and one primary reason to change.

This explainer was generated by Manning's knowledge graph from the text of ASP.NET Core in Action, 3e. The section links point to the parts of the book it draws on.