OWIN support for the Web API 2 and MVC 5 integrations in Autofac

Currently, in the both the Web API and MVC frameworks, dependency injection support does not come into play until after the OWIN pipeline has started executing. This is simply a result of the OWIN support being added to both frameworks after their initial release. In fact, the OWIN support in MVC does not yet extend to the self hosting scenario and is limited to plugging in OWIN middleware.

I had two primary objectives when creating these new OWIN packages for Autofac:

• Extend the Autofac lifetime scope from the OWIN pipeline into the MVC and Web API integrations
• Make dependency injection avaialable to OWIN middleware components

Due to the somewhat fragmented nature of the DI support in the ASP.NET stack, achieving this goal required a bit of trickery and black magic. For that reason the packages are currently pre-release and marked as alpha. Hopefully you can pull them from NuGet and help test them out.

Enabling DI for OWIN middleware

Lets start with a service interface for a logging dependency...

public interface ILogger
{
    void Write(string message, params object[] args);
}

...along with its rather unimaginative implementation (just imagine something more interesting).

public class Logger : ILogger
{
    public void Write(string message, params object[] args)
    {
        Debug.WriteLine(message, args);
    }
}

To enable dependency injection for OWIN middleware derive from OwinMiddleware as usual, and add any additional constructor dependencies after the constructor parameter for the next middleware in the chain. In the example below we are adding the ILogger dependency to the middleware.

public class LoggerMiddleware : OwinMiddleware
{
    private readonly ILogger _logger;

    public LoggerMiddleware(OwinMiddleware next, ILogger logger) : base(next)
    {
        _logger = logger;
    }

    public override async Task Invoke(IOwinContext context)
    {
        _logger.Write("Inside the 'Invoke' method of the 'LoggerMiddleware' middleware.");

        foreach (var pair in context.Environment)
            _logger.Write("Key: {0}, Value: {1}", pair.Key, pair.Value);

        await Next.Invoke(context);
    }
}

To wire this up we have to register the middleware and its dependencies with the container, and then enable the middleware dependency injection support via a call to UseAutofacMiddleware. This call should be the first piece of middleware registered with the IAppBuilder.

var builder = new ContainerBuilder();

builder.RegisterType<LoggerMiddleware>().InstancePerApiRequest();
builder.Register(c => new Logger()).As<ILogger>().InstancePerLifetimeScope();

app.UseAutofacMiddleware(container);

This particular extension method is included in the Autofac.Owin package which is shared by the Autofac.WebApi2.Owin and Autofac.Mvc5.Owin packages. You would not normally use this package by itself because of the second role the shared package plays. Not only is it responsible for adding middleware dependency injection support, it also creates an Autofac ILifetimeScope instance for each request early in the OWIN pipeline. You can access this lifetime scope using the GetAutofacLifetimeScope method on IOwinContext. The OWIN packages for the Web API and MVC integrations use this method to access the lifetime scope and make it available further along in the request.

Extending the lifetime scope to Web API

To enable the lifetime scope created during the OWIN request to extend into the Web API dependency scope call the UseAutofacWebApi method.

app.UseAutofacMiddleware(container);
app.UseAutofacWebApi(GlobalConfiguration.Configuration);

This will add a DelegatingHandler which extracts the ILifetimeScope from the IOwinContext. Once it has a hold of the ILifetimeScope an AutofacWebApiDependencyScope is created and added into the appropriate HttpRequestMessage property that Web API expects the IDependencyScope to be available from. The code below shows this task being performed during the SendAsync method of the handler.

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

    var owinContext = request.GetOwinContext();
    if (owinContext == null) return base.SendAsync(request, cancellationToken);

    var lifetimeScope = owinContext.GetAutofacLifetimeScope();
    if (lifetimeScope == null) return base.SendAsync(request, cancellationToken);

    var dependencyScope = new AutofacWebApiDependencyScope(lifetimeScope);
    request.Properties[HttpPropertyKeys.DependencyScope] = dependencyScope;

    return base.SendAsync(request, cancellationToken);
}

Notice that we still made a call to UseAutofacMiddleware. This is required because along with enabling the middleware DI support, this is also responsible for placing the lifetime scope in the OWIN context. I'm not totally happy with this dual responsibility but it does reduce the API surface area and means less IAppBuilder extensions are required. For now, if you don't want to use middleware DI support, don't register your middleware with the container.

Extending the lifetime scope to MVC

To enable the lifetime scope created during the OWIN request to extend into the MVC request call the UseAutofacMvc method on your IAppBuilder instance.

app.UseAutofacMiddleware(container);
app.UseAutofacMvc();

Just like with Web API we need to ensure that UseAutofacMiddleware has been called first. You only need to call this method once, even when using both the MVC and Web API integrations together. Remember, the call to UseAutofacMiddleware should be made as early as possible during the bootstrapping process.

In the MVC middleware we are also retrieving the ILifetimeScope from the IOwinContext, but in this case we are placing it in the HttpContext as is expected by the MVC integration. The code below shows this process directly implemented in a middleware method.

public static IAppBuilder UseAutofacMvc(this IAppBuilder app)
{
    return app.Use(async (context, next) =>
    {
        var lifetimeScope = context.GetAutofacLifetimeScope();
        var httpContext = CurrentHttpContext();

        if (lifetimeScope != null && httpContext != null)
            httpContext.Items[typeof(ILifetimeScope)] = lifetimeScope;

        await next();
    });
}

In case you are wondering, CurrentHttpContext is an internal property that returns a HttpContextBase allowing for mocking of the HTTP context during unit testing.

NuGet Packages

As mentioned earlier the packages are currently pre-release, so don't forget the -Preswitch on the command line, or the Include Prerelease option in the GUI. Please report any issues that you find on GitHub.

Autofac Web API OWIN Integration:

Install-Package Autofac.WebApi2.Owin -Pre

Autofac MVC 5 OWIN Integration:

Install-Package Autofac.Mvc5.Owin -Pre

Please note that you still need to configure the relevant dependency resolver for each integration: AutofacWebApiDependencyResolver for Web API, and AutofacDependencyResolverfor MVC.