Inject WebAPI UrlHelper into service using Autofac

Question

I have a service used by a few controllers in my WebAPI project. The service needs to generate URLs, so ideally it would get a UrlHelper via a constructor parameter.

public class MyService
{
    public MyService(UrlHelper urlHelper) { ... }
}

I'm using Autofac as my IoC container. How can I register UrlHelper in the container? It needs a HttpRequestMessage and I can't figure out how to get the "current" message.

Answer 1

Based on Darrel Miller's comment, I created the following:

A simple container class to hold a reference to the "current" HttpRequestMessage

public class CurrentRequest
{
    public HttpRequestMessage Value { get; set; }
}

A message handler that will store the current request

public class CurrentRequestHandler : DelegatingHandler
{
    protected async override System.Threading.Tasks.Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
    {
        var scope = request.GetDependencyScope();
        var currentRequest = (CurrentRequest)scope.GetService(typeof(CurrentRequest));
        currentRequest.Value = request;
        return await base.SendAsync(request, cancellationToken);
    }
}

In Global.asax, when configuring WebAPI, add the message handler.

GlobalConfiguration.Configuration.MessageHandlers.Insert(0, new CurrentRequestHandler());

Then, configure the Autofac container to let it construct UrlHelper, getting the current request from the CurrentRequest object.

var builder = new ContainerBuilder();
builder.RegisterType<CurrentRequest>().InstancePerApiRequest();
builder.Register(c => new UrlHelper(c.Resolve<CurrentRequest>().Value));
builder.RegisterType<MyService>();
builder.RegisterApiControllers(Assembly.GetExecutingAssembly());
...
container = builder.Build();

UrlHelper can then be injected into the MyService just like any other dependency.

Thanks to Darrel for pointing me in the right direction.