Cannot consume scoped service IMongoDbContext from singleton IActiveUsersService after upgrade to ASP.NET Core 2.0

Question

I updated a project to ASP.NET Core 2 today and I get the following error:

Cannot consume scoped service IMongoDbContext from singleton IActiveUsersService

I have the following registration:

services.AddSingleton<IActiveUsersService, ActiveUsersService>();
services.AddScoped<IMongoDbContext, MongoDbContext>();
services.AddSingleton(option =>
{
   var client = new MongoClient(MongoConnectionString.Settings);
   return client.GetDatabase(MongoConnectionString.Database);
})



public class MongoDbContext : IMongoDbContext
{
   private readonly IMongoDatabase _database;

   public MongoDbContext(IMongoDatabase database)
   {
      _database = database;
   }

   public IMongoCollection<T> GetCollection<T>() where T : Entity, new()
   {
      return _database.GetCollection<T>(new T().CollectionName);
   }
}

public class IActiveUsersService: ActiveUsersService
{

   public IActiveUsersService(IMongoDbContext mongoDbContext)
   {
      ...
   }
}

Why DI can't consume the service? All works fine for ASP.NET Core 1.1.

Answer 1

You can't use a service with a smaller lifetime. Scoped services only exist per-request, while singleton services are created once and the instance is shared.

Now only one instance of IActiveUsersService exists in the app. But it wants to depend on MongoDbContext, which is Scoped, and is created per-request.

You will have to either:

1. Make MongoDbContext a Singleton, or
2. Make IActiveUsersService Scoped, or
3. Pass MongoDbContext into the user service as a function argument

Answer 2

There are important differences between Scoped and Singleton services. The warning is there to bring this to light, and turning it off or switching around lifetimes indiscriminately to make it go away won't solve the problem.

Scoped services are created from an IServiceScope. One of its most important purposes is to ensure that any IDisposable services which are created in that scope are properly disposed when the scope itself is.

In ASP.NET Core, a service scope is automatically created for you on each incoming request, so you ordinarily don't need to worry about this. However, you can also create your own service scope; you just need to dispose of it yourself.

One way to do this is to:

• make your singleton service IDisposable,
• inject IServiceProvider,
• create and store an IServiceScope scope using the IServiceProvider.CreateScope() extension method,
• use that scope to create the the scoped service you need,
• dispose the service scope in the Dispose method.

services.AddSingleton<IActiveUsersService, ActiveUsersService>();
services.AddScoped<IMongoDbContext, MongoDbContext>();
services.AddSingleton(option =>
{
   var client = new MongoClient(MongoConnectionString.Settings);
   return client.GetDatabase(MongoConnectionString.Database);
})

public class MongoDbContext : IMongoDbContext
{
   private readonly IMongoDatabase _database;

   public MongoDbContext(IMongoDatabase database)
   {
      _database = database;
   }

   public IMongoCollection<T> GetCollection<T>() where T : Entity, new()
   {
      return _database.GetCollection<T>(new T().CollectionName);
   }
}

public class ActiveUsersService: IActiveUsersService, IDisposable
{
   private readonly IServiceScope _scope;

   public ActiveUsersService(IServiceProvider services)
   {
      _scope = services.CreateScope(); // CreateScope is in Microsoft.Extensions.DependencyInjection
   }

   public IEnumerable<Foo> GetFooData()
   {
       using (var context = _scope.ServiceProvider.GetRequiredService<IMongoDbContext>())
       {
           return context.GetCollection<Foo>();
       }
   }

   public void Dispose()
   {
       _scope?.Dispose();
   }
}

Depending on how you're using these and the scoped services you're consuming, you could instead do one of the following:

• create a single instance of the scoped service and use it for the life of the singleton; or
• store a reference to the (injected) root IServiceProvider, use it to create a new IServiceScope inside a using block every time you need a scoped service, and let the scope get disposed when the block exits.

Just keep in mind that any IDisposable services created from an IServiceScope will get automatically disposed when the scope itself does.

In short, don't just change around the lifetimes of your services to "make it work"; you still need to think about those and be sure they get disposed properly. ASP.NET Core handles the most common cases automatically; for others, you just need to do a bit more work.

Ever since C# 1.0 we have had using() blocks to ensure resources are disposed correctly. But using() blocks don't work when something else (the DI service) is creating those resources for you. That's where Scoped services come in, and using them incorrectly will lead to resource leaks in your program.

Answer 3

You can also add

.UseDefaultServiceProvider(options =>
                    options.ValidateScopes = false)

before .Build() in Program.cs file to disable the validation.

Try this only for development testing, ActiveUsersService is singleton and has a larger lifetime than MongoDbContext which is scoped and will not get disposed.