May 19, 2015

Resolve your DbContext as an interface using the ASP.NET 5 dependency injection framework

When developing ASP.NET applications, I prefer to have my database context implement an interface and then inject the interface, as this allows me to more easily mock the database context during unit tests.

So in a typical application, I would do something like this:

public interface IApplicationDbContext
{
    DbSet<Episode> Episodes { get; set; }
    DbSet<ApplicationUser> Users { get; set; }
    int SaveChanges();
    Task<int> SaveChangesAsync(CancellationToken cancellationToken);
}

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IApplicationDbContext
{
    public virtual DbSet<Episode> Episodes { get; set; }

    public ApplicationDbContext()
    {
		...
    }
}

And then inside my controllers, I will inject IApplicationDbContext:

public class EpisodesController : Controller
{
    private readonly IApplicationDbContext dbContext;

    public EpisodesController(IApplicationDbContext dbContext)
    {
        this.dbContext = dbContext;
    }

	...
}

For ASP.NET 4.5 I have been using Autofac, but for ASP.NET 5 I am using the built-in dependency injection mechanisms, so in your typical scenario, the registration of your database context and related services will look like this:

public void ConfigureServices(IServiceCollection services)
{
    ...

    // Add EF services to the services container.
    services.AddEntityFramework()
        .AddSqlServer()
        .AddDbContext<ApplicationDbContext>(options =>
            options.UseSqlServer(Configuration["Data:DefaultConnection:ConnectionString"]));

	...
}

Thanks to the ASP.NET 5 stack being open source we can look at the source code for the AddDbContext extension method (for beta 4 - maybe this changes in the future):

public virtual EntityFrameworkServicesBuilder AddDbContext<TContext>([CanBeNull] Action<DbContextOptionsBuilder> optionsAction = null)
    where TContext : DbContext
{
    _serviceCollection.AddSingleton(_ => DbContextOptionsFactory<TContext>(optionsAction));
    _serviceCollection.AddSingleton<DbContextOptions>(p => p.GetRequiredService<DbContextOptions<TContext>>());

    _serviceCollection.AddScoped(typeof(TContext), DbContextActivator.CreateInstance<TContext>);

    return this;
}

The method (in its current state) does not allow you to pass along separate genric types for the service and implementation, so I thought no problem, I will just add another line to register the service and implementation in the ConfigureServices method:

public void ConfigureServices(IServiceCollection services)
{
    ...

    // Add EF services to the services container.
    services.AddEntityFramework()
        .AddSqlServer()
        .AddDbContext<ApplicationDbContext>(options =>
            options.UseSqlServer(Configuration["Data:DefaultConnection:ConnectionString"]));

	// Register the service and implementation for the database context
	services.AddScoped<IApplicationDbContext, ApplicationDbContext>();

	...
}

This however did not turn out too well:

This is because the dependency injection framework will just try to create an instance ApplicationDbContext without all other configuration settings which is required - which the AddDbContext extensions method adds for you.

The solution was simple and that was to supply a lambda expression in the dependency injection registration which resolves an actual instance through the container and will therefore use the correct registration done by the AddDbContext method:

public void ConfigureServices(IServiceCollection services)
{
    ...

    // Add EF services to the services container.
    services.AddEntityFramework()
        .AddSqlServer()
        .AddDbContext<ApplicationDbContext>(options =>
            options.UseSqlServer(Configuration["Data:DefaultConnection:ConnectionString"]));

	// Register the service and implementation for the database context
	services.AddScoped<IApplicationDbContext>(provider => provider.GetService<ApplicationDbContext>());

	...
}

This configuration allows me to inject IApplicationDbContext into my controllers instead of ApplicationDbContext.