HttpClient client-side message handlers - asp.net

Question

The HttpClient custom Client Message Handlers insert themselves in the client-side pipeline when making web api calls. This info is from this article
The author says that these message handlers go into action as the request is being sent out and also when the response is received back from the server.
I understand these client message handler's role as request is being sent out from client to the Server because the sendAsync method provides the request object. One can add custom request headers, etc.
I do not understand the usefulness of these message handlers AFTER the response is received from the server because the sendAsync method does not provide access to the response object. So, I am not sure how these client side message handlers are useful on the return journey of the request.
I am obviously missing something here.

Answer 1

Actually you do get access to the response message inside the message handler. For example, in the following handler, I am logging outgoing request and incoming response. Now as long as I use this instance of HttpClient, all calls made through it write traces of requests and response.
HttpClient client = new HttpClient(new LoggingHandler(new HttpClientHandler()));
public class LoggingHandler : DelegatingHandler
{
public LoggingHandler(HttpMessageHandler innerHandler)
: base(innerHandler)
{
}
protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
Console.WriteLine("Request:");
Console.WriteLine(request.ToString());
if (request.Content != null)
{
Console.WriteLine(await request.Content.ReadAsStringAsync());
}
Console.WriteLine();
HttpResponseMessage response = await base.SendAsync(request, cancellationToken);
Console.WriteLine("Response:");
Console.WriteLine(response.ToString());
if (response.Content != null)
{
Console.WriteLine(await response.Content.ReadAsStringAsync());
}
Console.WriteLine();
return response;
}
}