Dependency Injection for EF with Autofac on Web API

Question

I have a housekeeping services that I am dynamically injecting when saving an entity in my DbContext.

protected override DbEntityValidationResult ValidateEntity(DbEntityEntry entityEntry, IDictionary<object, object> items)
{
    var t = entityEntry.Entity.GetType();

    // Housekeep
    var castEntityEntry = castDbEntityEntryMethod
        .MakeGenericMethod(new[] { t })
        .Invoke(entityEntry, null);
    housekeepEntityMethod
        .MakeGenericMethod(new[] { t })
        .Invoke(this, new[] { castEntityEntry });

    return base.ValidateEntity(entityEntry, items);
}

private void HousekeepEntity<TEntity>(DbEntityEntry<TEntity> entity)
    where TEntity : class
{
    var housekeeper = DbConfiguration.DependencyResolver.GetService<IHousekeeper<TEntity>>();
    if (housekeeper != null)
    {
        housekeeper.Maintain(entity);
    }
}

I had been passing IComponentContext around everywhere so I could Resolve the type there, but with EF6 I hoped to use the dependency injection:

private class AutofacDbDependencyResolver : AutofacWebApiDependencyResolver, IDbDependencyResolver
{
    public AutofacDbDependencyResolver(ILifetimeScope container)
        : base(container)
    {
    }

    public object GetService(System.Type type, object key)
    {
        using (var scope = this.GetRequestLifetimeScope())
        {
            if (scope == null)
            {
                return null;
            }

            object service;
            scope.TryResolve(type, out service);
            return service;
        }
    }

    public System.Collections.Generic.IEnumerable<object> GetServices(System.Type type, object key)
    {
        using (var scope = this.GetRequestLifetimeScope())
        {
            if (scope == null)
            {
                return System.Linq.Enumerable.Empty<object>();
            }

            object service;
            if (scope.TryResolve(type, out service))
            {
                return new object[] { service };
            }
            else
            {
                return System.Linq.Enumerable.Empty<object>();
            }
        }
    }
}

When setting up container:

System.Data.Entity.DbConfiguration.Loaded += (s, e) =>
    e.AddDependencyResolver(new AutofacDbDependencyResolver(container), overrideConfigFile: false);

However my lifetime scopes return null. How can I get the current API request which calls this action?

Steven Steven · Accepted Answer · 2014-07-30T07:00:44

Prevent using the service locator anti-pattern inside your DbContext. It doesn't matter that EF6 now allows you to resolve it, it is still an anti-pattern. Instead simply use dependency injection or allow this housekeeping to be added by hooking onto an event. For instance, you can write a partial class for your entities:

public partial class MyEntities
{
    public event Action<DbContext> SavingChanges = _ => { };

    public override int SaveChanges()
    {
        this.SavingChanges(this);

        return base.SaveChanges();
    }
}

This way you can simply hook onto the SavingChanges event that will get raised when someone calls SaveChanges:

builder.Register<MyEntities>(context =>
{
    var db = new MyEntities();
    db.SavingChanges += context.Resolve<IHouseKeepingService>().HousekeepEntities;
    return db;
}).InstancePerLifetimeScope();

Dependency Injection for EF with Autofac on Web API

1 Answers