Alexa.Tip – Using Dependency Injection with Handlers in .NET

In this Alexa.Tip series, we explore some little bits of code that can make your life easier in developing Alexa Skills in many languages including C# + .NET, node.jS + TypeScript, Kotlin, etc. We look at concepts, design patterns, and implementations that developers might not be aware of, and how they can be applied to voice application development, best practices, and more!

In this post, we explore some more best practices in developing Alexa Skills in C# whether you are using an ASP.NET Core API or an AWS Lambda. This time, we talk about taking our Handler Registration Pattern to the next level by using dependencies in them such as EF DbContexts or really any other service you want to inject.

Check out all the raw source code for this post, and more here: https://github.com/SuavePirate/Alexa.Tips.Net

If you haven’t read up on how to use the Handler Registration Pattern, take a look at my earlier post here: Alexa.Tip – Using Handler Registration Pattern in .NET

The short version is that we use this pattern of registering IHandler implementations to handle different types of requests that our skill receives, regardless of whether we are using AWS Lambdas or ASP.NET Core APIs.

Building Handlers with Dependencies

So we’ve seen samples of simple Handlers that use static responses simply using the ResponseBuilder from the Alexa.NET Library, but now let’s take it to the next logical step and start to get some data from a database. In this case, I am using Entity Framework, and therefore need to access my DbContext to get data.

To do this, we should inject the DbContext, in this case a SampleMessageDbContext into the constructor of the SampleFactHandler, then use it locally in the HandleAsync() method to get a single message.

SampleFactHandler.cs

public class SampleFactHandler : GenericHandler, IHandler
{
    private readonly SampleMessageDbContext _context;
    public SampleFactHandler(SampleMessageDbContext context)
    {
        _context = context;
    }
    public override string IntentName => "SampleMessageIntent";

    public override Type RequestType => typeof(IntentRequest);

    public override async Task<SkillResponse> HandleAsync(SkillRequest request)
    {
        // just grab one as an example
        var message = await _context.SampleMessages.FirstOrDefaultAsync();
        return ResponseBuilder.Tell(message?.Content ?? "I don't have any messages for you.");
    }
}

Although this simple example just grabs the first row from the db for the SampleMessages table, you could imagine some more complex data logic applied here to find the proper response we want for the given request. I also added some quick null handling just to clean it up for a real sample.

Registering Handlers with Dependencies

Now that we have our SampleFactHandler, we need to register it to our ICollection, but also construct it using the SampleMessageDbContext. There are a couple ways to do this. For simple situations, you just need to register the DbContext in your Startup, then grab it from the ServiceCollection when creating the List:

Startup.cs

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        // add other dependencies first
        services.AddDbContext<SampleMessageDbContext>(options =>
        {
            options.UseInMemoryDatabase("InMemoryDbForTesting");
        });

        // Register your handlers here!
        services.AddScoped<ICollection<IHandler>>(s =>
        {
            // get the db context to inject in the handlers that are dependent
            var dbContext = s.GetRequiredService<SampleMessageDbContext>();
            return new List<IHandler>
            {
                new SimpleLaunchHandler(),
                new DogFactHandler(),
                new SampleFactHandler(dbContext)
            };
        });

        services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseHsts();
        }

        app.UseHttpsRedirection();
        app.UseMvc();
    }
}

Alternatively, you could create a class whose responsibility is to hold onto the handlers and construct that list with all the IHandlers. Something like this:

IHandlerContainer.cs

public interface IHandlerContainer
{
    ICollection<IHandler> Handlers { get; }
}

HandlerContainer.cs

public class HandlerContainer : IHandlerContainer 
{
    public ICollection<IHandler> Handlers { get; private set; }
    public HandlerContainer(SimpleLaunchHandler launch, DogFactHandler dogFact, SampleFactHandler sampleFact)
    {
        Handlers = new List<IHandler> { launch, dogFact, sampleFact }
    }
}

With this container, we update our Startup to look something like this:

 public void ConfigureServices(IServiceCollection services)
    {
        // add other dependencies first
        services.AddDbContext<SampleMessageDbContext>(options =>
        {
            options.UseInMemoryDatabase("InMemoryDbForTesting");
        });
        services.AddScoped<SimpleLaunchHandler>();
        services.AddScoped<DogFactHandler>();
        services.AddScoped<SampleFactHandler>();
        services.AddScoped<IHandlerContainer, HandlerContainer>();

        services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
    }

Then our Controller would consume the IHandlerContainer rather than a flat ICollection to look like this:

SimpleAlexaController.cs

[Route("[controller]")]
public class SimpleAlexaController : Controller
{
    private readonly IHandlerContainer _handlerContainer;
    public SimpleAlexaController(IHandlerContainer handlerContainer)
    {
        _handlerContainer = handlerContainer;
    }
    [HttpPost]
    public async Task<SkillResponse> HandleRequest([FromBody]SkillRequest request)
    {
        var viableHandler = _handlerContainer.Handlers.FirstOrDefault(h => h.CanHandle(request));
        return await viableHandler.HandleAsync(request);
    }
}

I think this little pattern helps cleanup the Startup without needing to explicitly pull dependencies out of the ServiceCollection in order to build the IHandlers. Which do you prefer?

What’s next?

In future posts, we’ll take a look at building on these types of handlers with things like:

  • Well written Unit Tests
  • Full Integration Tests
  • Advanced Contextual driven handlers

If there’s enough interest in this pattern and the tools I’m building around it, let me know in GitHub or Twitter and I’ll work on getting them into properly libraries and NuGet packages 🙂

Check out more Alexa Developer Tips here: https://alexdunn.org/tag/alexa/


If you like what you see, don’t forget to follow me on twitter @Suave_Pirate, check out my GitHub, and subscribe to my blog to learn more mobile and AI developer tips and tricks!

Interested in sponsoring developer content? Message @Suave_Pirate on twitter for details.


voicify_logo
I’m the Director and Principal Architect over at Voicify. Learn how you can use the Voice Experience Platform to bring your brand into the world of voice on Alexa, Google Assistant, Cortana, chat bots, and more: https://voicify.com/


Xamarin.Tip – Mvvm Light and Dependency Injection

Inversion of Control and Dependency Injection are some design principles that help make our applications more flexible and scalable. They both help us separate our implementations and make it easy to substitute drastic changes to our implemented data or business logic whether it be for writing unit tests or product improvement.

Xamarin is a platform where IoC and DI fit extremely well. I’ve talked about this concept a few other times in both my blogs and videos about the Onion Architecture in Xamarin as well as how to call Platform Specific code from a Portable Class Library. You can find those posts and videos here:

  1. Onionizing Xamarin Part 6
  2. [VIDEO] Xamarin.Tips: Calling Platform-Specific Code from a PCL (Dependency Injection)

In this post, I want to talk about using DI with Mvvm Light at a VERY basic level.

First, let’s define an interface for a service we might use:

IUserService.cs

public interface IUserService
{
    Task<User> GetCurrentUserAsync();
}

Now let’s create two different implementations. One that will be the service used in the application and the other that will be used for testing.

UserService.cs

public class UserService : IUserService
{
    // makes a call to a web api to get a user
    public async Task<User> GetCurrentUserAsync()
    {
        using (var client = new HttpClient())
        {
            var response = await client.GetAsync("https://mywebapi.mydomain/api/currentuser");
            var content = await response.Content.ReadAsStringAsync();
            return JsonConvert.DeserializeObject<User>(content);
        }
    }
}

TestUserServices.cs

public class TestUserService : IUserService
{
    public Task<User> GetCurrentUserAsync()
    {
        return Task.FromResult(new User { Name = "Test User" });
    }
}

Now we need a ViewModel that will use this service. We define a private readonly IUserService and then inject the implementation that we want in the constructor of the ViewModel.

CurrentUserViewModel.cs

public class CurrentUserViewModel : ViewModelBase
{
    // use the interface as the service and inject the implementation in the constructur
    private readonly IUserService _userService;
    private User _user;

    public User User
    {
        get
        {
            return _user;
        }
        set
        {
            Set(ref _user, value);
        }
    }

    public CurrentUserViewModel(IUserService userService)
    {
        _userService = userService;
    }

    public async Task UpdateUserAsync()
    {
        User = await _userService.GetCurrentUserAsync();
    }
}

Now let’s define an IoCConfig that handles registering dependencies and implementations.

IoCConfig.cs

public class IoCConfig
{
    public IoCConfig()
    {
        // use SimpleIoc from MvvmLight as our locator provider
        ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
    }

    // register the real implementation
    public void RegisterServices()
    {
        SimpleIoc.Default.Register<IUserService, UserService>();
    }

    // register the test implementation
    public void RegisterTestServices()
    {
        SimpleIoc.Default.Register<IUserService, TestUserService>();
    }

    // register the view model
    public void RegisterViewModels()
    {
        SimpleIoc.Default.Register<CurrentUserViewModel>();
    }
}

Now that we can register our Services as well as our ViewModels, the dependency resolver from SimpleIoc can retrieve an instance of CurrentUserViewModel with whichever version of IUserService is registered depending on whether we call RegisterServices or RegisterTestServices.

Now we can retrieve our instance of the CurrentUserViewModel by calling

var currentUserViewModel = ServiceLocator.Current.GetInstance<CurrentUserViewModel>();

MvvmLight recommends using a ViewModelLocator to get the instance of your ViewModels:

ViewModelLocator.cs

public class ViewModelLocator
{
    private readonly IoCConfig _iocConfig;
    public CurrentUserViewModel CurrentUser
    {
        get
        {
            return ServiceLocator.Current.GetInstance<CurrentUserViewModel>();
        }
    }

    public ViewModelLocator()
    {
        _iocConfig = new IoCConfig();
        _iocConfig.RegisterServices();
        //_iocConfig.RegisterTestServices();
        _iocConfig.RegisterViewModels();
    }

}

It’s recommended to either create your ViewModelLocator at the app start up, or if you’re using Xamarin.Forms, register it as a Resource in your App.xaml

<Application ...     xmlns:locator="clr-namespace:YOUR_LOCATOR_LOCATION">
    <Application.Resources>
        <ResourceDictionary>
            <locator:ViewModelLocator x:Key="Locator"/>
        </ResourceDictionary>
    </Application.Resources>
</Application>

Now in your XAML pages, you can automatically wire up your view model.

MainPage.xaml

<ContentPage ...     BindingContext="{Binding Source={StaticResource Locator}, Path=CurrentUser}"     Title="{Binding User.Name}">
...
</ContentPage>

In order to change to your testing data, you can just switch which call to your IoCConfig is made for registering your dependency without having to make any changes to any of your other layers or UI!

If you like what you see, don’t forget to follow me on twitter @Suave_Pirate, check out my GitHub, and subscribe to my blog to learn more mobile developer tips and tricks!

Interested in sponsoring developer content? Message @Suave_Pirate on twitter for details.