Implementing Custom Message Handlers In ASP.NET Web API | ASP.NET Web API Reference Guide | ASP.NET Web API Tutorial

Steps to write a custom message handler: derive from System.Net.Http.DelegatingHandler and override theSendAsync method. This method has the following signature:

Task<HttpResponseMessage> SendAsync(
    HttpRequestMessage request, CancellationToken cancellationToken);

The method takes an HttpRequestMessage as input and asynchronously returns an HttpResponseMessage. A typical implementation does the following:

1. Process the request message.
2. Call base.SendAsync to send the request to the inner handler.
3. The inner handler returns a response message. (This step is asynchronous.)
4. Process the response and return it to the caller.

Here is a trivial example:

public class MessageHandler1 : DelegatingHandler
{
    protected async override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {
        Debug.WriteLine("Process request");
        // Call the inner handler.
        var response = await base.SendAsync(request, cancellationToken);
        Debug.WriteLine("Process response");
        return response;
    }
}