Decorating ASP.NET Web API IHttpController

Question

I'm trying to wrap Web API controllers (IHttpController implementations) with decorators, but when I do this, Web API throws an exception, because somehow it is expecting the actual implementation.

Applying decorators to controllers is a trick I successfully apply to MVC controllers and I obviously like to do the same in Web API.

I created a custom IHttpControllerActivator that allows resolving decorated IHttpController implementations. Here's a stripped implementation:

public class CrossCuttingConcernHttpControllerActivator : IHttpControllerActivator {
    private readonly Container container;
    public CrossCuttingConcernHttpControllerActivator(Container container) {
        this.container = container;
    }

    public IHttpController Create(HttpRequestMessage request, 
        HttpControllerDescriptor controllerDescriptor, Type controllerType)
    {
        var controller = (IHttpController)this.container.GetInstance(controllerType);

        // Wrap the instance in one or multiple decorators. Note that in reality, the 
        // decorator is applied by the container, but that doesn't really matter here.
        return new MyHttpControllerDecorator(controller);
    }
}

My decorator looks like this:

public class MyHttpControllerDecorator : IHttpController {
    private readonly IHttpController decoratee;
    public MyHttpControllerDecorator(IHttpController decoratee) {
        this.decoratee = decoratee;
    }

    public Task<HttpResponseMessage> ExecuteAsync(
        HttpControllerContext controllerContext,
        CancellationToken cancellationToken)
    {
        // this decorator does not add any logic. Just the minimal amount of code to
        // reproduce the issue.
        return this.decoratee.ExecuteAsync(controllerContext, cancellationToken);
    }
}

However, when I run my application and request the ValuesController, Web API throws me the following InvalidCastException:

Unable to cast object of type 'WebApiTest.MyHttpControllerDecorator' to type 'WebApiTest.Controllers.ValuesController'.

Stacktrace:

at lambda_method(Closure , Object , Object[] )
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass13.<GetExecutor>b__c(Object instance, Object[] methodParameters)
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.Execute(Object instance, Object[] arguments)
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.<>c__DisplayClass5.<ExecuteAsync>b__4()
at System.Threading.Tasks.TaskHelpers.RunSynchronously[TResult](Func`1 func, CancellationToken cancellationToken)

It's just as if Web API supplies us with the IHttpController abstraction but skips it and still depends on the implementation itself. This would of course be a severe violation of the Dependency Inversion principle and make the abstraction utterly useless. So I'm probably doing something wrong instead.

What I'm I doing wrong? How can I happily decorate my API Controllers?

Answer 1

I would say, that the natural, designed way how to achieve this behaviour in ASP.NET Web API is with the Custom Message Handlers / Delegation Handlers

For example I do have this DelegationHandler in place

public class AuthenticationDelegationHandler : DelegatingHandler
{
    protected override System.Threading.Tasks.Task<HttpResponseMessage> 
        SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // I. do some stuff to create Custom Principal
        // e.g.
        var principal = CreatePrincipal();
        ...

        // II. return execution to the framework            
        return base.SendAsync(request, cancellationToken).ContinueWith(t =>
        {
            HttpResponseMessage resp = t.Result;
            // III. do some stuff once finished
            // e.g.:
            // SetHeaders(resp, principal);

            return resp;
        });
    }

And this is how to inject that into the structure:

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.MessageHandlers.Add(new AuthenticationDelegationHandler());