I have not had much experience using Dependency Injection but I am trying to use it in one of my projects. When I try to add a second parameter to the constructor of my I get an error "Cannot consume scoped service 'CarbonService' from singleton..." Not sure why I get this error or what I am missing. Thanks for your help
I would like my timer class to have access to the CarbonService object.
Startup.cs:
public void ConfigureServices(IServiceCollection services)
{
services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
services.AddDbContext<CarbonDBContext>(o => o.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
services.AddHostedService<ValidUntilTimerService>();
services.AddScoped<ICarbonService, CarbonService>(); // I HAVE TRIED USING SINGLETON BUT SAME ERROR
services.AddLogging(loggingBuilder => loggingBuilder.AddSerilog(dispose: true));
}
ValidUntilTimerService.cs:
// ADDING THE SECOND PARAMETER TO THIS CONSTRUCTOR CAUSES THE ERROR.
public class ValidUntilTimerService : IHostedService, IDisposable
{
private readonly ILogger _logger;
private Timer _timer;
private ICarbonService _carbonService;
public ValidUntilTimerService(ILogger<ValidUntilTimerService> logger, ICarbonService carbonService)
{
_logger = logger;
_carbonService = carbonService;
}
...
}
CarbonService.cs:
public interface ICarbonService
{
WattTime ReadCarbon();
void SaveCarbon(int carbonIndex);
}
public class CarbonService : ICarbonService
{
private IConfiguration _configuration;
private readonly CarbonDBContext _dbcontext;
public CarbonService(IConfiguration configuration, CarbonDBContext dbContext)
{
_configuration = configuration;
_dbcontext = dbContext;
}
public WattTime ReadCarbon()
{
var wt = new WattTime();
...
return wt;
}
public void SaveCarbon(int carbonIndex)
{
...
}
}
IHostedServices are registered as singletons. You are injecting ICarbonService into ValidUntilTimerService, which means you are injecting a scoped service into a singleton service.
If we consider the lifetimes of these two types of services:
Scoped services are created once per request. This means, when a scoped service is instantiated, it stays alive until the end of the request.
When singleton service gets instantiated, it stays alive until the app shuts down.
we realize that it doesn't make much sense to consume scoped service from a singleton service. This is what would happen in that situation:
When singleton service (ValidUntilTimerService in your case) gets instantiated, its scoped dependency (ICarbonService in your case) also gets instantiated and injected to the singleton service. When the request is completed, the singleton service stays alive, but the scoped service gets disposed. Now your singleton service ended up with a "dead" dependency (_carbonService field in your case).
The above scenario is not possible. It is just a "what if" scenario. The point is, you cannot consume a service that is created per request from a service that can be created without request (e.g. upon app startup).
Now, that explains the cause of your issue, but let's see what you can do to solve it.
POSSIBLE SOLUTION
You can use IServiceScopeFactory to create your own scope inside of the ValidUntilTimerService:
public class ValidUntilTimerService : IHostedService, IDisposable
{
private readonly ILogger _logger;
private Timer _timer;
private IServiceScopeFactory _serviceScopeFactory;
public ValidUntilTimerService(ILogger<ValidUntilTimerService> logger, IServiceScopeFactory serviceScopeFactory)
{
_logger = logger;
_serviceScopeFactory = serviceScopeFactory;
}
// ...
using (var scope = _serviceScopeFactory.CreateScope())
{
ICarbonService carbonService = scope.ServiceProvider.GetService(typeof(ICarbonService));
// ...
}
// ...
}
Related
I have a class that derives from BackgroundService (IHostedService) for running background tasks. This will be added to my services using builder.Services.AddHostedService<BackgroundTaskService>()
BackgroundService's task runs for the entire duration of the web application, checking for queued data to process.
My question is, how do I instantiate an instance of DbContext from this code?
I could have the BackgroundTaskService constructor accept a DbContext. But wouldn't that keep the DbContext open forever?
And how else could I instantiate it without duplicating all the code to scan my settings file for the connection string, etc.?
The recemmended approach is to inject IDbContextFactory<TContext> as described in the following article: Using a DbContext factory (e.g. for Blazor)
Some application types (e.g. ASP.NET Core Blazor) use dependency injection but do not create a service scope that aligns with the desired DbContext lifetime. Even where such an alignment does exist, the application may need to perform multiple units-of-work within this scope. For example, multiple units-of-work within a single HTTP request.
In these cases, AddDbContextFactory can be used to register a factory for creation of DbContext instances.
public void ConfigureServices(IServiceCollection services)
{
services.AddDbContextFactory<ApplicationDbContext>(
options =>
options.UseSqlServer(#"Server=(localdb)\mssqllocaldb;Database=Test"));
}
Then in your controller:
private readonly IDbContextFactory<ApplicationDbContext> _contextFactory;
public MyController(IDbContextFactory<ApplicationDbContext> contextFactory)
{
_contextFactory = contextFactory;
}
public void DoSomething()
{
using (var context = _contextFactory.CreateDbContext())
{
// ...
}
}
You can use scope service factory. Check here for reference.
Here you have an example:
// Injection
public class DataApi : BackgroundService
{
private readonly ILogger<DataApi> logger;
private readonly IServiceScopeFactory scopeFactory;
public DataApi(ILogger<DataApi> _logger, IConfiguration _cfg, IServiceScopeFactory _sSF)
{
logger = _logger;
scopeFactory = _sSF;
// e.g. data from appsettings.json
// var recovery = _cfg["Api:Recovery"];
}
// ...
// Usage
protected async Task DataCollector()
{
logger.LogInformation("Collector");
using (var scope = scopeFactory.CreateScope())
{
var db = scope.ServiceProvider.GetRequiredService<MyDbContext>();
var myList = await db.MyEntity
.AsNoTracking()
.Where(t => t.active)
.ToListAsync();
if (myList.Count == 0)
{
logger.LogInformation("Empty...");
return;
}
// logic...
}
await Task.CompletedTask;
}
I have a scoped Context that is being accessed from transient service through method.
This transient service is being injected into singleton service.
Will my scoped Context become singleton or will it stay scoped?
public class Context : IContext
{
public string CorrelationId { get; set; }
public Context(string id)
{
CorrelationId = id;
}
}
Context accessor:
internal class RequestContextRegistrator : IRequestContextRegistrator
{
private IContext context;
public IContext RegisterContext(IContext context)
{
this.context = context;
return context;
}
public IContext Get()
{
return context ?? new Context()
{
CorrelationId = context.CorrelationId
};
}
}
And Singleton object:
public class QueueSender<TCommand>
{
private readonly IRequestContextRegistrator provider;
public QueueSender(IRequestContextRegistrator provider)
{
this.provider = provider;
}
public async Task Send(TCommand command)
{
var context = provider.Get();
var message = PrepareServiceBusMessage(command, userAgent, context?.CorrelationId);
}
}
The whole idea is to be able to pass around context id that is unique to the particular "request". The request is not coming from dotnet controller, it comes from queue receiver class.
Or to paraphrase it, how deep does this conversion to singleton goes for tree of dependency injections.
Do not resolve a scoped service from a singleton. It may cause the service to have incorrect state when processing subsequent requests. It's fine to:
Resolve a singleton service from a scoped or transient service.
Resolve a scoped service from another scoped or transient service.
please see this link Service lifetime
Will my scoped Context become singleton or will it stay scoped?
It will stay scoped.
Your singleton instance will have a RequestContextRegistrator injected which will in turn have a Context injected; this Context instance will exist until your app terminates, as the singleton will preserve its reference, however, any other class that requires an IContext will have a new Context injected.
Most solutions I've found for this question involve using an IServiceScopeFactory to inject scoped services into a hosted service. I don't want to do this because it makes testing difficult. It'd be nice to inject the services I need into my hosted service using an IServicesCollection, just like Startup.cs. Ideally something like this:
# Program.cs
...
.ConfigureServices((hostContext, services) =>
{
services.AddDbContext<MyContext>();
services.AddSingleton<IOtherInjectedService, OtherInjectedService>();
services.AddScoped<MyService>(); // <-- how does this work?
services.AddHostedService<MyHostedService>();
});
# MyHostedService.cs
public class MyHostedService : IHostedService, IDisposable
{
private readonly MyService _svc;
public MyHostedService(MyService svc)
{
_svc = svc;
}
public void DoWork() {
_svc.Work();
}
}
# MyService.cs
public class MyService
{
private readonly MyContext _context;
private readonly IOtherInjectedService _otherService;
public void Work()
{
_otherService.Foo(_context);
}
}
This falls apart when I try to configure the services in Program.cs. What I see is this error:
Cannot consume scoped service 'MyService' from singleton 'Microsoft.Extensions.Hosting.IHostedService'.
Which makes sense -- a singleton cannot consume a scoped service. But it looks like there is no way to make IHostedService scoped. Does that mean there is no way to inject dependencies without using a scope factory? I hope not! It makes testing difficult, to say the least.
What is the right way to inject services into a hosted service without using an IServiceScopeFactory?
Let's say I have my own class QueueListener<TService, TPayload> inherited from BackgroundService. It opens TCP connection and listens incoming messages. On each message I would like to initialize service of TService type and pass deserialized from JSON instance of TPayload to it. TService is going to be registered as Transient, so it means to be lightweight and stateless as a handler for payload have to be (in my current task). For this purpose I am going to inject IServiceProvider in constructor of my QueueListener and create a scope on each message it receives. Does it sounds like a plan or am I overengineering? I want to avoid TService is singleton as well.
Documentation says:
It's dangerous to resolve a scoped service from a singleton. It may cause the service to have incorrect state when processing subsequent requests.
But I am not completely sure what does it means. There is no way to inject scoped service in BackgroundService because it has Singleton lifetime. Do they warn me to stop doing things like I do?
UPD #1
I explain why I suppose to create scope on each message. The idea behind that is to prevent listener to be blocked by message processing and to provide other developers possibility to create their own handlers and do some stuff on received message. Other developers can create database connections for instance while processing and I want it to be closed and released when handling is done.
Register TService as scoped and create a new scope per message. Then resolve TService from created scope. Just read Consuming a scoped service in a background task
You can write it like this:
services.AddHostedService<MyBackgroundService>();
services.AddScoped<IScopedServicePerMessage, ScopedServicePerMessage>();
...
public class MyBackgroundService : BackgroundService
{
private readonly IServiceProvider _sp;
public MyBackgroundService(IServiceProvider sp)
{
_sp = sp;
}
protected override Task ExecuteAsync(CancellationToken stoppingToken)
{
DoWork(stoppingToken);
return Task.CompletedTask;
}
private void DoWork(CancellationToken stoppingToken)
{
while(true)
{
var msg = GetNextMessage();
using (var scope = _sp.CreateScope())
{
var servicePerMessage = scope.ServiceProvider.GetRequiredService<IScopedServicePerMessage>();
servicePerMessage.Handle(msg);
}
}
}
...
}
Regarding this:
It's dangerous to resolve a scoped service from a singleton. It may
cause the service to have incorrect state when processing subsequent requests.
It's about the case when you inject scoped service (ef core dbcontext, for instance) directly into singleton. It's not your case.
The documentation is referring to injecting a scoped service into a singleton service. Since the injection happens at the construction of the singleton object, the scoped service would be provided at that time. This will effectively increase the lifetime of the scoped service to that of a singleton service. This is dangerous because a scoped service lifetime is often chosen explicitly to ensure that the object gets disposed quickly again.
The most common example would be a database context which owns a database connection; you want to make sure that you free up this database connection as soon as possible to free up the resources. But if you injected the context into a singleton service, it would never get disposed.
That however does not mean that there is no way to consume scoped services within a singleton service. This is done by having the singleton service create a service scope from which it can then retrieve singleton services. It’s important though that this service scope is supposed to be short-lived. So take the example from ASP.NET Core itself, where a service scope is created for every request, and do something similar. For example in your case, you could do it for every incoming message if that makes sense for your application.
To create a service scope, you should inject an IServiceScopeFactory; you can then create a scope with it like this:
public async Task Process(TPayload payload)
{
using (var scope = _serviceScopeFactory.CreateScope())
{
var service = scope.GetService<TService>();
await service.Process(payload);
}
}
This pattern is strictly only necessary if you need to consume scoped services. You could resolve all other services directly without creating a scope. If you can reuse the same service instance to process all payloads, you could also inject the service as a singleton (same as registering it as transient but resolving it only once). If you need a fresh instance for every payload, then consider creating a scope even if it isn’t strictly necessary.
First, transient services are not scoped services. Transient services are usually externally owned by your code and are created each time they are resolved from the container. Container does not cache transient services.
TService is going to be registered as Transient ... For this purpose I am going to inject IServiceProvider in constructor of my QueueListener and create a scope on each message it receives.
You do not need a scope for resolving transient services. Even if you create a scope, the scope still does not manage / own transient services. That, for example, ending the lifetime of the scope does not end lifetime of transient services.
You could simply use the IServiceProvider injected in QueueListener to resolve TService. And each TService resolved should be already like what you want
lightweight and stateless as a handler for payload
With regards to
Documentation says:
What the document says might not be relevant now since you are not using scoped services. But in case you want to know the reason:
It's dangerous to resolve a scoped service from a singleton.
Singleton is a special kind of scope. Singleton services are created and cached within a "root" scope of the container, which is essentially the container itself.
If you resolve scoped service from singleton, the lifetime / scope where the service instance is resolved and cached is likely to be the "root" scope. This leads to a problem where the scoped service instance being cached inside the container, and shared across multiple client requests.
This is dangerous, because scoped services are supposed to be
Scoped lifetime services (AddScoped) are created once per client request (connection).
I don't wanted my singleton class depending on the IServiceProvider directly.
So I've used a custom factory to accomplish this goal.
May this code example help others:
public class Startup
{
// ...
public void ConfigureServices(IServiceCollection services)
{
services.AddScoped<IScopedBar, ScopedBar>();
services.AddSingleton<IScopedServiceFactory<IScopedBar>, ScopedServiceFactory<IScopedBar>>(
(provider) => {
var scope = provider.CreateScope();
var service = scope.ServiceProvider.GetRequiredService<IScopedBar>();
return new ScopedServiceFactory<IScopedBar>(() => new ScopedService<IScopedBar>(scope, service));
});
services.AddSingleton<ISingletonFoo, SingletonFoo>();
}
// ...
}
public interface ISingletonFoo
{
void DoSomethingUsingScopedServices();
}
public class SingletonFoo : ISingletonFoo
{
private readonly IScopedServiceFactory<IScopedBar> _barFactory;
public SingletonFoo(IScopedServiceFactory<IScopedBar> barFactory)
{
_barFactory = barFactory;
}
public void DoSomethingUsingScopedServices()
{
using var scopedService = _barFactory.CreateService();
scopedService.Service.DoSomething();
}
}
public interface IScopedBar
{
void DoSomething();
}
public class ScopedBar : IScopedBar
{
public void DoSomething()
{
// Do something
}
}
public interface IScopedService<T> : IDisposable
{
T Service { get; }
}
public interface IScopedServiceFactory<T>
{
IScopedService<T> CreateService();
}
public class ScopedService<T> : IScopedService<T>
{
private readonly IDisposable _scope;
public ScopedService(IDisposable scope, T service)
{
_scope = scope;
Service = service;
}
public T Service { get; }
public void Dispose()
{
_scope.Dispose();
}
}
public class ScopedServiceFactory<T> : IScopedServiceFactory<T>
{
private readonly Func<IScopedService<T>> _serviceFactory;
public ScopedServiceFactory(Func<IScopedService<T>> serviceFactory)
{
_serviceFactory = serviceFactory;
}
public IScopedService<T> CreateService()
{
return _serviceFactory();
}
}
Here is my code to consume scoped service:
public interface IScopedResolver<T> where T: class
{
TResult Resolve<TResult>(Func<T, TResult> dataFactory);
Task<TResult> ResolveAsync<TResult>(Func<T, Task<TResult>> dataFactory);
}
Implement class:
public class ScopedResolver<T> : IScopeResolver<T> where T: class
{
private readonly IServiceProvider _provider;
public ScopedResolver(IServiceProvider provider)
{
_provider = provider;
}
public TResult Resolve<TResult>(Func<T, TResult> dataFactory)
{
using IServiceScope scope = _provider.CreateScope();
var service = scope.ServiceProvider.GetRequiredService<T>();
return dataFactory(service);
}
public async Task<TResult> ResolveAsync<TResult>(Func<T, Task<TResult>> dataFactory)
{
using var scope = _provider.CreateScope();
var service = scope.ServiceProvider.GetRequiredService<T>();
return await dataFactory(service);
}
}
Register at startup:
services.AddSingleton(typeof(IScopedResolver<>), typeof(ScopedResolver<>));
using ScopedResolve:
public class ServiceA
{
private readonly IScopedResolver<DbContext> _context;
public ServiceA(IScopedResolver<DbContext> context)
{
_context = context;
}
public async Task<List<ClassOne>> GetListAsync()
{
return await _context.ResolveAsync(async s => await s.Set<ClassOne>().ToListAsync());
}
}
I am creating a ASP.NET Core web application. I am using a Repository through a library project. I reference it in the web application project.
The repository interface is as below:
public interface IPushNotificationRepository
{
IQueryable<PushNotification> Notifications
{
get;
}
IQueryable<Client> Clients
{
get;
}
void Add(PushNotification notification);
void Add(Client client);
void AddRange(IList<PushNotification> notifications);
bool AddIfNotAlreadySent(PushNotification notification);
void UpdateDelivery(PushNotification notification);
bool CheckIfClientExists(string client);
Client FindClient(int? id);
void Update(Client client);
void Delete(Client client);
}
Within the repository I inject the db context
public class PushNotificationRepository : IPushNotificationRepository
{
private readonly PushNotificationsContext _context;
public PushNotificationRepository(PushNotificationsContext context)
{
_context = context;
}
}
The configure services of the start up class is as below:
public void ConfigureServices(IServiceCollection services)
{
services.AddControllersWithViews();
services.AddSingleton<IPushNotificationRepository, PushNotificationRepository>();
services.AddDbContextPool<PushNotificationsContext>(options =>
options.UseSqlServer(Configuration.GetConnectionString("PushNotificationsConnection")));
}
In the controller class I consume the repository:
public class ClientsController : Controller
{
//private readonly PushNotificationsContext _context;
private readonly IPushNotificationRepository _pushNotificationRepository;
public ClientsController(IPushNotificationRepository pushNotificationRepository)
{
_pushNotificationRepository = pushNotificationRepository;
}
}
The repository classes are in a separate library project which is referenced by the web application project. The error I receive is:
System.AggregateException: 'Some services are not able to be
constructed (Error while validating the service descriptor
'ServiceType:
Services.Messaging.Data.Abstract.IPushNotificationRepository Lifetime:
Singleton ImplementationType:
Services.Messaging.Data.PushNotificationRepository': Cannot consume
scoped service 'Services.Messaging.Data.PushNotificationsContext' from
singleton
'Services.Messaging.Data.Abstract.IPushNotificationRepository'.)'
Would really appreciate some advise on this
A singleton cannot reference a Scoped instance. The error message is clear.
Cannot consume scoped service
'Services.Messaging.Data.PushNotificationsContext' from singleton
PushNotificationsContext is considered as a scoped service. You should almost never consume scoped service or transient service from a singleton. You should also avoid consuming transient service from a scoped service. Consuming scoped services it's a good practice to inject what you need, it gets cleaned up after the request automatically.
Either
services.AddTransient < IPushNotificationRepository, PushNotificationRepository>();
or
services.AddScoped< IPushNotificationRepository, PushNotificationRepository>();
will work fine, but check your design. Maybe this is not the behaviour you are looking for.
services.AddDbContext<PushNotificationsContext>() registers the PushNotificationsContext as a service with ServiceLifetime.Scoped which means that your PushNotificationsContext is created per web request. It is disposed when request is completed.
You could inject IServiceScopeFactory which is singleton into your repository, then create a new scope using CreateScope() and request the PushNotificationsContext service from that scope
public class PushNotificationRepository : IPushNotificationRepository
{
IServiceScopeFactory _serviceScopeFactory;
public PushNotificationRepository(IServiceScopeFactory serviceScopeFactory)
{
_serviceScopeFactory = serviceScopeFactory;
}
public void Add(PushNotification notification);
{
using (var scope = _serviceScopeFactory.CreateScope())
{
var context = scope.ServiceProvider.GetRequiredService<PushNotificationsContext>();
//other logic
}
}
}
Refer to c# - DataContext disposed in ASP.NET Core scheduler