I am trying to understand the difference between AddSingleton, AddScoped and AddTransient. There are lots of explanations but I can't understand them because I don't understand when an HTTP request is within the same scope
So, there's always a "root container" that's shared by all consumers in the AppDomain or running process. Child containers are then created for each HTTP request (in ASP.NET Core, for each HttpContext which encompasses HttpRequest and HttpResponse). (Note that child containers can be created for other reasons too, but that's outside this answer's concern).
Singleton services are only constructed once, usually only by the root container. They're like the Singleton-pattern in OOP (where a class can only be instantiated once), except in this case you can still manually create multiple instances, but the DI container will only ever create 1 instance itself.
You can use OOP Singletons with DI containers by returning the OOP singleton instance from a service factory method.
Transient services are always created when they're requested - they're meant to be short-lived services. Some containers will call IDisposable.Dispose on all transient services it creates, others will not (as they expect the consumer to dispose of them, check with your container's policies).
Request-scoped services can be implemented differently by different container systems - but a common approach I see is that at the start of each HTTP request (when a new HttpContext is created) a child-container is created (a child-container inherits the registrations of its parent) and then all of the objects it creates (often as singletons, but only in that child container) are then disposed (if applicable) when the HTTP request ends (when the HttpContext is destroyed, after the HTTP response has been sent to the client and the response ended).
Disregarding ASP.NET entirely - let's pretend we have our own HTTP server program with its own DI container:
public class HttpServer
{
private readonly IContainer rootContainer;
public HttpServer()
{
this.rootContainer = RegisterServices( new ContainerBuilder() ).Build();
}
private static IContainerBuilder RegisterServices( IContainerBuilder services )
{
return services
.RegisterSingleton<ISystemClock,BiosClock>()
.RegisterSingleton<MySingleton>( factory: () => MySingleton.Instance )
.RegisterTransient<IDbConnection>( factory: () => new SqlConnection() )
.RegisterRequest<RequestTracingService>();
}
public void OnHttpRequest( Socket socket )
{
HttpContext context = new HttpContext();
context.RequestContainer = this.rootContainer.CreateChildContainer();
try
{
// hand-off the `context` object to code that reads the request, does processing, and then writes the response
}
finally
{
context.RequestContainer.Dispose(); // <-- this disposes of any objects created by RequestContainer during the processing of the request, without touching any objects created by `rootContainer`.
}
}
}
Related
I'm having a huge problem with the configuration/dependency injection of an application.
I have a singleton class added through DI with AddSingleton, that has in its constructor a IRequestClient, that is scoped because
busConfigurator.AddRequestClient()
which among other things, has the same effect as AddScoped.
When I start the app, it says
"Cannot consume scoped service 'MassTransit.IRequestClient`1[...]' from singleton '...'.)"
Which absolutely makes sense.
The weirdest thing is that I have another app set up the exact same way, but it just works and I would really like for that class to remain singleton.
My colleague and I spent an entire day trying to find the differences between the two applications, but they are virtually the same in their configurations, so we are having trouble in understanding why one works while the other doesn't.
I'm not entirely sure on what details could be important to better define the problem, so feel free to ask.
We've looked all around the internet trying to find a solution, but it was always "Change singleton to transient", but that's not an option, first because it HAS to be a singleton, otherwise it wouldn't make sense in our app, as that thing is what caches lots of date from our db so we can't just go around keeping on collecting heaps of data, second because the first app works with singleton, not with transient and we'd like to keep it that way
// This method is called in Main()
private static void ConfigureMassTransit(IServiceCollection services)
{
services.AddMassTransit(busConfigurators =>
{
busConfigurators.AddRequestClient<ICacheRepository>();
busConfigurators.AddConsumers(typeof(Program).GetTypeInfo().Assembly);
busConfigurators.UsingRabbitMq((context, cfg) =>
{
cfg.Host(new Uri($"rabbitmq://{Config.Settings.RabbitMq_Host}"), hostConfigurator =>
{
hostConfigurator.Username(Config.Settings.RabbitMq_User);
hostConfigurator.Password(Config.Settings.RabbitMq_Password);
});
cfg.ReceiveEndpoint("myApp", e =>
{
e.ConfigureConsumers(context);
});
});
});
// CacheRepository
public class CacheRepository : ICacheRepository
{
private readonly IClient Client;
public CacheRepository(ICacheRepository client, ILogger<CacheRepository> logger)
{
this.client = client;
this.logger = logger;
}
}
When a dependency is scoped, the implication is that a new instance is needed for each scope (which is usually an incoming HTTP request or message.) It implies that the instance should not be re-used for multiple requests.
If you have a singleton that depends on that scoped dependency, that singleton will be created using an instance of that dependency (the request client.) Because that singleton "lives forever," so does the instance of the request client it contains.
The result is that the request client is not supposed to be re-used across different scopes, but now it is. One instance is used forever.
A likely solution is to modify the class that depends on that client so that it doesn't need to be a singleton. You mentioned that it has to be a singleton because it caches data.
How does it cache data? Does it do so by storing data in a private field? If so, perhaps you could make that field static. Now the class instance isn't re-used, but those fields are shared between instances. (Verify that interaction with those fields is thread safe if they may be accessed concurrently.)
Or if there's some other cache mechanism, you could move that into its own dependency and make that a singleton.
Then your class can be scoped. It will depend on the singleton cache, always using the same instance. It will also depend on the scoped request client, using a new instance for each scope.
You could inject IServiceProvider instead, and create a scope when the singleton needs to perform a request. That way, you're sticking to the expected use of the request client.
await using var scope = provider.CreateAsyncScope();
var client = scope.ServiceProvider.GetRequiredService<IRequestClient<T>>();
await client.GetResponse(...);
Given the following classes
public class RWRepository<TEntity, TEntityId> : IRWRepository<TEntity, TEntityId>
and
internal sealed class EagerLoadingRWRepository<TEntity>
: RWRepository<TEntity, Guid>
, IEagerLoadingRWRepository<TEntity>
I am registering these in my DI container like this (where service is IServiceCollection)
services.AddScoped(typeof(IRWRepository<,>), typeof(RWRepository<,>));
services.AddScoped(typeof(IEagerLoadingRWRepository<>), typeof(EagerLoadingRWRepository<>));
Say I inject both of these into 2 different handlers, both within the same scope. How I can ensure that both handlers will use the same object instance for each injected interface?
I.e. if both interfaces are injected, then both should use the same instance of EagerLoadingRWRepository, which inherits from RWRepository
This will be very cumbersome to do with MS.DI (ASP.NET Core's built-in DI Container), because it requires you to make each closed registration explicitly.
For instance:
services.AddScoped<IEagerLoadingRWRepository<Person>, EagerLoadingRWRepository<Person>>();
services.AddScoped<IRWRepository<Person, Guid>>(
c => (IRWRepository<Person, Guid>)c.GetRequiredService<IEagerLoadingRWRepository<Person>>());
services.AddScoped<IEagerLoadingRWRepository<Order>, EagerLoadingRWRepository<Order>>();
services.AddScoped<IRWRepository<Order, Guid>>(
c => (IRWRepository<Order, Guid>)c.GetRequiredService<IEagerLoadingRWRepository<Order>>());
services.AddScoped<IEagerLoadingRWRepository<Shipment>, EagerLoadingRWRepository<Shipment>>();
services.AddScoped<IRWRepository<Shipment, Guid>>(
c => (IRWRepository<Shipment, Guid>)c.GetRequiredService<IEagerLoadingRWRepository<Shipment>>());
How I can ensure that both handlers will use the same object instance?
Scoped Lifetime works exactly as you describe. You don't need to ensure anything.
Microsoft Docs
https://learn.microsoft.com/en-us/dotnet/core/extensions/dependency-injection#scoped
For web applications, a scoped lifetime indicates that services are created once per client request (connection). Register scoped services with AddScoped.
In apps that process requests, scoped services are disposed at the end of the request.
Lifetimes:
Transient objects are always different. The transient OperationId value is different in the request handler and in the middleware.
Scoped objects are the same for a given request but differ across each new request.
Singleton objects are the same for every request.
I have a factory class in a Net Core 3 console app which needs to be able to resolve against a DI container at runtime:
public class OptionFactory : IOptionFactory
{
private readonly IServiceProvider _svcProvider;
public OptionFactory( IServiceProvider svcProvider )
{
_svcProvider = svcProvider;
}
public IOption<T>? CreateOption<T>( params string[] keys )
{
// code eliminated for brevity
try
{
return retVal = _svcProvider.GetRequiredService<Option<T>>();
}
catch( Exception e )
{
return null;
}
}
}
I'm using Autofac to define the DI container and then "assign" it to IServiceProvider via new AutofacServiceProvider( builder.Build() ) in a provider class:
public class TestServiceProvider
{
public static IServiceProvider Instance { get; private set; }
static TestServiceProvider()
{
var builder = new ContainerBuilder();
builder.RegisterType<OptionFactory>()
.As<IOptionFactory>()
.SingleInstance();
// code omitted for brevity
Instance = new AutofacServiceProvider( builder.Build() );
}
}
I'm unclear about how to register IServiceProvider itself with the DI container so that it can be injected into the constructor. Is that even possible? It seems a little self-referential, which could be problematic.
All the examples I've seen online call for referencing back to the Autofac IContainer itself, (or to TestServiceProvider.Instance in my example). I can do that, but it would end tie my library to a concrete service provider class. Which I think I'd like to avoid if I can.
I realize injecting IServiceProvider is considered an anti-pattern by some/many, although others deem it acceptable in a factory class because the factory is "simply" extending the DI container. I'm open to other approaches which don't rely on a factory class, provided they allow me to create concrete instances of open generic types at runtime.
You have a couple of options (no pun intended 😃).
Easiest: Call builder.Populate() with an empty collection
The Autofac.Extensions.DependencyInjection package (which you're using, since you have AutofacServiceProvider) has an extension method ContainerBuilder.Populate() which handles registering stuff from an IServiceCollection and auto-registering the AutofacServiceProvider. You could call that method with an empty service collection and it'll work.
builder.Populate(Enumerable.Empty<ServiceDescriptor>());
This will get you exactly the thing you're looking for. However, there's an alternative to consider...
Alternative: Use ILifetimeScope
If it doesn't matter whether your OptionFactory is tied to Autofac, you can inject ILifetimeScope. Autofac has the current lifetime scope auto-registered, so this will work:
public OptionFactory(ILifetimeScope scope)
{
// scope is whatever lifetime scope the
// factory itself came from - if that's the
// root container, then the scope is the
// container
}
The benefit here is you'll get the richer resolve options Autofac offers without any extra work. The drawback would be you're tied to Autofac at this level, which may or may not matter.
Beware!
It may just be your example, but there's something important to know if you're resolving directly from the root container the way the example shows:
You could easily end up with a big memory leak.
Autofac holds on to all IDisposable instances it resolves so they can be safely disposed when the lifetime scope is disposed. If you are resolving from the container, that means any IDisposable will be held onto until the container itself is disposed, which, for most, is the lifetime of the application. That means - hypothetically - every resolution could be adding just a tiny little bit of memory that won't be disposed until the container is disposed. Memory leak.
For this reason we recommend always resolving from a nested lifetime scope rather than from the container. In a web app, that request-level lifetime scope is perfect because it disappears after a request. In an example like this, it's up to you and your app code to determine the best way to integrate lifetime scopes.
And, of course, if you're definitely, 100% guaranteed never resolving anything IDisposable, no worries.
Controller
public class LocationsController : ApiController
{
private readonly IMediator _mediator;
public LocationsController(IMediator mediator)
{
_mediator = mediator;
}
public IEnumerable<Place> Get()
{
return _mediator.Send(new GetLatestMapData<Place>());
}
}
On first request of Get() action, the Handler is instantiated by SimpleInjector and executed correctly.
On the second request (F5 in browser for e.g.), it fails with :
Handler was not found for request of type ....
Container or service locator not configured properly or handlers not registered with your container.
and inner exception of:
Cannot access a disposed object.
Object name: 'The ThreadLocal object has been disposed.'
OWIN Startup
public class Startup
{
public void Configuration(IAppBuilder app)
{
// SimpleInjector
var container = CompositionRoot.CreateContainer();
var config = GlobalConfiguration.Configuration;
config.DependencyResolver = new SimpleInjectorWebApiDependencyResolver(container);
// Routing
config.MapHttpAttributeRoutes();
config.Routes.MapHttpRoute("DefaultApi", "api/{controller}/{id}",
new { id = RouteParameter.Optional });
config.EnsureInitialized();
app.UseWebApi(config);
}
}
SimpleInjector IPackage for WebAPI project
public class Installer : IPackage
{
public void RegisterServices(Container c)
{
c.Options.DefaultScopedLifestyle = new WebApiRequestLifestyle();
c.RegisterWebApiControllers(GlobalConfiguration.Configuration);
}
}
I think what's happening is the Handler is correctly created, and then disposed after the first request.
Now, I don't know why, but on subsequent requests, the Handler isn't re-created. I know this because if I change the WebApiRequestLifestyle to 'not dispose when scope ends', it works for every request:
c.Options.DefaultScopedLifestyle = new WebApiRequestLifestyle(false
/*disposeInstanceWhenScopeEnds*/);
Questions
Should I keep the disposeInstanceWhenScopeEnds parameter set to false?
If not, what is the correct solution?
I see this has been solved before by creating a LifetimeScopeDecorator... however, surely this functionality is already provided by the SimpleInjector WebApi integration library? What am I missing?
(And thank you for reading)
This link provides good guidance on dependency resolution and using the IDependencyResolver / IDependencyScope Interfaces.
Immediately you will see that they touch on life spans which tend to get a little tricky.
This section in particular is interesting:
Dependenecy Scope and Controller Lifetime
Controllers are created per request. To manage object lifetimes,
IDependencyResolver uses the concept of a scope.
The dependency resolver attached to the HttpConfiguration object has
global scope. When Web API creates a controller, it calls BeginScope.
This method returns an IDependencyScope that represents a child scope.
Web API then calls GetService on the child scope to create the
controller. When request is complete, Web API calls Dispose on the
child scope. Use the Dispose method to dispose of the controller’s
dependencies.
Conventionally bootstrapping a service would occur once during the app start-up and as you know resolve any dependencies at that time. Only when the worker process was shutting down (no activity, for example) would this then invoke dispose.
Ordinarily I think it is quite normal for resolved instances to remain for the life cycle unless it is imperative that they are destroyed after use. But the example given explains that we must correctly dispose once the request is complete. So I would recommend that you dispose of your instances correctly using the examples provided as guidance.
This helped me when working with IoC and WebApi. I hope this helps!
You need to arrange your Lifetime scoped
Code:
container.Options.DefaultScopedLifestyle = new WebApiRequestLifestyle();
container.Options.LifestyleSelectionBehavior = new WebApiInjectionLifestyle();
internal class WebApiInjectionLifestyle : ILifestyleSelectionBehavior
{
public Lifestyle SelectLifestyle(Type serviceType, Type implementationType)
{
return Lifestyle.Scoped;
}
}
More Detail
https://simpleinjector.readthedocs.io/en/latest/lifetimes.html
In Microsoft Unity IoC, if I call Resolve<SomeType>(), can I guarantee that the object returned is the one that was created during the current session?
For example, three users sign on, and let's say that the object of SomeType that gets created in the container has different values for each user. Will a call to Resolve return the object that was created for the current user? Or would it do something stupid like return the last one that was created?
I'm having troubles testing this myself due to some environment problems and I need to check something in soon, so if someone could answer this it would be very helpful!
Edit
Forgive me for I am very new to Unity, but based on what I read here, it seems like I should be able to register objects in the container with a unique name and retrieve them by that name. So, wouldn't I be able to use a session ID or some other value that persists within a session to retrieve my object?
Oh wow, lifetime management using Unity in am MVC app. Where do I start?
First of all, session singletons are not really possible as there is no ASP.NET system that will guarantee that the same instance will be used between requests in the same session. The session can mimic the same object persisted within the session by serializing and deserializing it between requests.
Transient instances - i.e. simple registrations without lifetime management specification are sufficient 99% of the time. This implies that an instance of registered type will be created every time it is needed.
It is very rarely that you need instances to live throughout the lifetime of the request. However when you need those, you really need those. A connection to a DB is a perfect candidate for this. Request singletons, on the other hand are much easier to create and manage.
The most elegant solution is to use Unity's child container feature. A child container can be created at the beginning of the request, disposed at the end of the request (as an added bonus it will dispose all ContainerControlledLifetimeManager instances).
When creating a child container, all registrations are still available from the parent container, so you need to register request specific stuff with the child container.
Here is pseudo-code to get this working:
private void Application_Start() {
_parentContainer = new UnityContainer();
//creates a transient registration, available at any point in the app.
_parentContainer.RegisterType<IParentIntf, ParentIntfImpl>();
ControllerBuilder.Current.SetControllerFactory(new ServiceLocatorControllerFactory());
}
private void Application_BeginRequest() {
var childContainer = _parentContainer.CreateChildContainer();
//registers a request "singleton"
//This registration is a type registration, an instance of RequestInterfaceImpl
//will be created when needed and then kept in the container for later use.
childContainer.RegisterType<IRequestInterface,RequestInterfaceImpl>(new ContainerControlledLifetimeManager());
//save the child container in the context, so we can use it later
HttpContext.Items["childContainer"] = childContainer;
}
private void Application_EndRequest() {
//dispose the child container
((IUnityContainer)HttpContext.Items["childContainer"]).Dispose();
}
One other thing that needs to be done is to override the Controller Factory to use the child container to create controllers. Controller are the first point of entry into the application and they could simply take a dependency on other components in their constructor.
public class UnityControllerFactory : DefaultControllerFactory {
#region IControllerFactory Members
public override IController CreateController(System.Web.Routing.RequestContext requestContext, string controllerName) {
IController controller;
controllerName = controllerName.ToLower();
var container = ((IUnityContainer)HttpContext.Items["childContainer"])
if(container.IsRegistered<IController>(controllerName))
controller = container.Resolve<IController>(controllerName);
else
controller = base.CreateController(requestContext, controllerName) ;
return controller;
}
}
The default behaviour will be to return a new instance for each resolve call, this isn't what you want.
It would be possible to create and resolve the same instance within a session, but there is no built in support as far as I know. You would have to write your own lifetime manager, and then use this when registering your type.
There is a lifetime manager that can do per thread instances, but this isn't useful for sessions as threads will get re-used, and resolve would need to also work across multiple requests to be truly session-scoped.
It's entirely possible that someone has written a lifetime manager for this.