AddHttpMessageHandler

AddHttpMessageHandler is a method in the .NET framework used to configure the `HttpClient` pipeline. It allows developers to register custom handlers that will process HTTP requests and responses before they are sent or received. These handlers are typically instances of `DelegatingHandler`.

When you call AddHttpMessageHandler, you are essentially adding a step to the chain of responsibility for HTTP communication. Each `DelegatingHandler` in the pipeline has the opportunity to examine, modify, or even short-circuit a request or response. This is particularly useful for implementing cross-cutting concerns such as logging, authentication, retry logic, or modifying headers.

The handlers are executed in the order they are added. A `DelegatingHandler` can invoke the next handler in the chain by calling `base.SendAsync`. If a handler does not call the next handler, the request or response processing stops at that point.

This extensibility makes `HttpClient` highly customizable and robust. For example, you might add a logging handler first to record all outgoing requests, followed by an authentication handler to attach credentials, and finally a retry handler to gracefully manage transient network failures. The `HttpClientBuilder` class, obtained from `IHttpClientFactory`, is commonly used to access and chain these handlers.