FerranGuardia/unity-di

Unity DI Container (5.x)

CRITICAL: This is the Unity DI container (Unity.Container), NOT Microsoft.Extensions.DependencyInjection (ASP.NET Core). Different API, different lifetime models, different registration patterns.

Container Setup

using Unity;
using Unity.Lifetime;

IUnityContainer container = new UnityContainer();

// Interface → Implementation
container.RegisterType<IService, MyService>();

// With lifetime
container.RegisterType<ILogger, FileLogger>(new ContainerControlledLifetimeManager());

// Existing instance
container.RegisterInstance<IAppConfig>(new AppConfig { ConnStr = "..." });

// Resolve
IService service = container.Resolve<IService>();

Constructor selection: Unity picks the constructor with the longest parameter list by default. Override with InjectionConstructor:

container.RegisterType<ILogger, MockLogger>(
    new ContainerControlledLifetimeManager(),
    new InjectionConstructor()  // force parameterless constructor
);

Lifetime Managers

| Manager | Behavior | Use For | |---------|----------|---------| | TransientLifetimeManager | Default. New instance every Resolve() | Stateless services | | ContainerControlledLifetimeManager | Singleton within container + children | App-wide singletons | | SingletonLifetimeManager | Global singleton across entire container tree | Truly global state | | HierarchicalLifetimeManager | Per-scope singleton (child gets its own) | Per-request in ASP.NET | | PerResolveLifetimeManager | Singleton within one Resolve() call graph | Shared dependency in one resolution | | ContainerControlledTransientManager | Transient but container tracks + disposes | Disposable transients | | ExternallyControlledLifetimeManager | Weak reference only | Externally managed objects | | PerThreadLifetimeManager | One per thread | Thread-local state |

Per-Request Pattern (ASP.NET)

// HierarchicalLifetimeManager + child containers = per-request scope
container.RegisterType<IDbContext, MyContext>(new HierarchicalLifetimeManager());

The MVC/Web API integration creates a child container per request. HierarchicalLifetimeManager gives each child its own instance.

ASP.NET MVC 5 Integration

Package: Unity.Mvc (v5.11.1)

// App_Start/UnityConfig.cs
using Unity;
using Unity.Mvc;
using System.Web.Mvc;

public static class UnityConfig
{
    public static void RegisterComponents()
    {
        var container = new UnityContainer();

        container.RegisterType<IProductRepository, ProductRepository>();
        container.RegisterType<IOrderService, OrderService>(
            new HierarchicalLifetimeManager());

        DependencyResolver.SetResolver(new UnityDependencyResolver(container));
    }
}

ASP.NET Web API 2 Integration

Package: Unity.AspNet.WebApi (v5.11.2)

// App_Start/UnityWebApiActivator.cs
using Unity;
using Unity.AspNet.WebApi;
using System.Web.Http;

public static class UnityWebApiActivator
{
    public static void Start()
    {
        var container = new UnityContainer();

        container.RegisterType<IProductService, ProductService>();
        container.RegisterType<IUserRepository, UserRepository>(
            new HierarchicalLifetimeManager());

        GlobalConfiguration.Configuration.DependencyResolver =
            new UnityDependencyResolver(container);
    }
}

Combined MVC + Web API

Share the same UnityContainer but set two separate resolvers:

var container = new UnityContainer();
// ... registrations ...

// MVC resolver
System.Web.Mvc.DependencyResolver.SetResolver(new Unity.Mvc.UnityDependencyResolver(container));

// Web API resolver (different class, different namespace)
GlobalConfiguration.Configuration.DependencyResolver =
    new Unity.AspNet.WebApi.UnityDependencyResolver(container);

Injection Patterns

Constructor Injection (preferred)

public class OrderService : IOrderService
{
    private readonly IRepository _repo;
    private readonly ILogger _logger;

    // Unity resolves both automatically
    public OrderService(IRepository repo, ILogger logger)
    {
        _repo = repo;
        _logger = logger;
    }
}

Property Injection

using Unity;

public class OrderService : IOrderService
{
    [Dependency]
    public ILogger Logger { get; set; }

    [Dependency("special")]    // Named dependency
    public ICache Cache { get; set; }
}

// Or via registration
container.RegisterType<IOrderService, OrderService>(
    new InjectionProperty("Logger"));

Named Registrations

container.RegisterType<INotifier, EmailNotifier>("email");
container.RegisterType<INotifier, SmsNotifier>("sms");

// Resolve specific
INotifier email = container.Resolve<INotifier>("email");

// Resolve ALL named (does NOT include default unnamed)
IEnumerable<INotifier> all = container.ResolveAll<INotifier>();

// Inject specific named dependency
container.RegisterType<AlertService>(
    new InjectionConstructor(new ResolvedParameter<INotifier>("sms")));

Registration by Convention

using Unity.RegistrationByConvention;

container.RegisterTypes(
    AllClasses.FromLoadedAssemblies()
        .Where(t => t.Namespace?.StartsWith("MyApp.Services") == true),
    WithMappings.FromMatchingInterface,  // IFoo → Foo
    WithName.Default,
    WithLifetime.Transient
);

Common Pitfalls

| Pitfall | Problem | Solution | |---------|---------|----------| | Singleton holds per-request dependency | Captive dependency — scoped object lives forever | Use same or longer lifetime for dependencies | | IDisposable with TransientLifetimeManager | Container doesn't track/dispose transients | Use ContainerControlledTransientManager or dispose manually | | Forgetting to dispose child containers | Scoped singletons leak | Ensure MVC/WebAPI integration disposes per-request containers | | Sharing InjectionConstructor instances | Runtime errors | Each RegisterType needs its own InjectionConstructor | | ContainerControlledLifetimeManager + IDisposable | Object never disposed until container dies | Call container.Dispose() on app shutdown | | v5.2.1 singleton sharing change | Two interfaces mapping to same impl no longer share instance | Register impl separately, then map both interfaces |

v5.2.1 Breaking Change

// Before 5.2.1: these share one MockLogger instance
container.RegisterType<ILogger, MockLogger>(new ContainerControlledLifetimeManager());
container.RegisterType<IDebugger, MockLogger>(new ContainerControlledLifetimeManager());

// After 5.2.1: these create SEPARATE instances. To share:
container.RegisterType<MockLogger>(new ContainerControlledLifetimeManager());
container.RegisterType<ILogger, MockLogger>();
container.RegisterType<IDebugger, MockLogger>();

Checklist

• [ ] HierarchicalLifetimeManager used for per-request services (DbContext, UnitOfWork)
• [ ] ContainerControlledLifetimeManager used for true singletons only
• [ ] Both MVC and Web API resolvers set if using both frameworks
• [ ] container.Dispose() called in Application_End
• [ ] No captive dependencies (singleton holding scoped reference)
• [ ] Constructor injection preferred over [Dependency] property injection
• [ ] No reuse of InjectionConstructor instances across registrations