More Information for WebAPI MediaTypeFormatter

[ErikXu]

I want to know more about the registration for MediaTypeFormatter while the introduction on http://autofac.readthedocs.org/en/latest/integration/webapi.html#supported-services is too simple. Could you please help to solve the questions for me?
#1 How to add a custom media formatter to the HttpConfiguration with dependence?
#2 How to replace a existed media formatter in the HttpConfiguration with dependence?

[tillig]

Web API doesn't pass the set of MediaTypeFormatter instances attached to HttpConfiguration.Formatters through dependency injection - it's a static set of entities.

If you want to integrate DI with your root set of media type formatters, it's going to be something custom you'll need to build. There is no guidance we have available.

One idea (not guidance, just an idea) is to build the container and then resolve the set of formatters.

var builder = new ContainerBuilder();
builder.RegisterType<MyFirstFormatter>().As<MediaTypeFormatter>();
builder.RegisterType<MySecondFormatter>().As<MediaTypeFormatter>();
builder.RegisterType<MyDependency>().As<IDependency>();
var container = builder.Build();

foreach(var formatter in container.Resolve<IEnumerable<MediaTypeFormatter>())
{
  config.MediaTypeFormatters.Add(formatter);
}

Note these would be singletons and would be resolved from the root container - you wouldn't get per-request dependencies or anything like that. For the global level, I don't see any easy way around that.

Now, I do see that the MediaTypeFormatter class has a method GetPerRequestFormatterInstance which, by default, just returns the current instance.

You might be able to do something with that. You get the HttpRequestMessage in there, so you could do something like this:

public class MyFormatter : MediaTypeFormatter
{
  public MyFormatter()
  {
    // Default behavior. Probably(?) won't be used.
  }

  public MyFormatter(IDependency dependency)
  {
    // The "real" constructor.
  }

  public override MediaTypeFormatter GetPerRequestFormatterInstance(
    Type type,
    HttpRequestMessage request,
    MediaTypeHeaderValue mediaType)
  {
    // Run a resolve to get the dependencies injected.
    return request.GetDependencyScope(typeof(MyFormatter)) as MyFormatter;
  }
}

You'd still have to register a "placeholder" for the media type formatter with the static collection, but hypothetically this would add DI to it.

// Use the "default" constructor just to get the formatter
// into the collection.
config.MediaTypeFormatters.Add(new MyFormatter());

Now, as mentioned in the docs, you can use the [AutofacControllerConfiguration] attribute to get per-controller media type formatters. Here's one of our unit tests showing that in action.

[Test]
public void FormattersCanBeResolvedPerControllerType()
{
    var builder = new ContainerBuilder();
    var formatter1 = new Mock<MediaTypeFormatter>().Object;
    var formatter2 = new Mock<MediaTypeFormatter>().Object;
    builder.RegisterInstance(formatter1).InstancePerApiControllerType(typeof(TestController));
    builder.RegisterInstance(formatter2).InstancePerApiControllerType(typeof(TestController));
    var container = builder.Build();
    var configuration = new HttpConfiguration {DependencyResolver = new AutofacWebApiDependencyResolver(container)};
    var settings = new HttpControllerSettings(configuration);
    var descriptor = new HttpControllerDescriptor(configuration, "TestController", typeof(TestController));
    var attribute = new AutofacControllerConfigurationAttribute();

    attribute.Initialize(settings, descriptor);

    Assert.That(settings.Formatters.Count, Is.EqualTo(6));
    Assert.That(settings.Formatters.Contains(formatter1), Is.True);
    Assert.That(settings.Formatters.Contains(formatter2), Is.True);
}

That works on a per-controller basis, but that's the only actual integration we have for DI in media type formatters.

Hopefully between the various ideas above it should unblock you. If not, it might be a good question to open on StackOverflow to see if the larger community has ideas.

[ErikXu]

The second one is a good solution for me. Thanks for your reply.