Using multiple delegating handlers in a web API call

Question

I have a web API call that needs to make use of 2 delegating handlers, but I never seem to get in to the second handler. The route is registered with the handlers. I am not allowed to set the handlers in the global config.

 DelegatingHandler[] handlers = new DelegatingHandler[2];
        handlers[0] = new HandlerA();
        handlers[1] = new HandlerB();

        RouteTable.Routes.MapHttpRoute(
            name: "TestAPI",
            routeTemplate: "api/{controller}",
            handler: HttpClientFactory.CreatePipeline(
                innerHandler: new HttpClientHandler(),
                handlers: handlers
            ),
            defaults: new { controller = "test"},
            constraints: null
        );

HandlerA triggers a breakpoint without further problems.

 protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) {
        if (request == null)
            throw new ArgumentNullException("request");

        //Breakpoint is hit
        return base.SendAsync(request, cancellationToken);
    }

I'd expected to arrive in the HandlerB but it never does, it goes straight to the API controller. Handler A doesn't do anything to the request.

score 1 · Accepted Answer · answered Jun 06 '16 at 15:32

I found the answer to my problem in this blog post: http://byterot.blogspot.nl/2012/05/aspnet-web-api-series-messagehandler.html

Instead of giving the API route the array of handlers, the first handler had the second handler set as inner handler.

DelegatingHandler[] handlers = new DelegatingHandler[2];
handlers[0] = new HandlerA();
handlers[1] = new HandlerB();
handlers[0].InnerHandler = handlers[1];

RouteTable.Routes.MapHttpRoute(
        name: "TestAPI",
        routeTemplate: "api/{controller}",
        handler: handlers[0],
        defaults: new { controller = "test"},
        constraints: null
    );

Using multiple delegating handlers in a web API call

1 Answers1