How to consume a Scoped service from a Singleton?

Question

How should I inject (using .NET Core's built-in dependency injection library, MS.DI) a DbContext instance into a Singleton? In my specific case the singleton is an IHostedService?

What have I tried

I currently have my IHostedService class take a MainContext (deriving from DbContext) instance in the constructor.

When I run the application I get:

Cannot consume scoped service 'Microsoft.EntityFrameworkCore.DbContextOptions' from singleton 'Microsoft.Extensions.Hosting.IHostedService'.

So I tried to make the DbContextOptions transient by specifying:

services.AddDbContext<MainContext>(options =>
    options.UseSqlite("Data Source=development.db"),
    ServiceLifetime.Transient);

in my Startup class.

But the error remains the same, even though, according to this solved Github issue the DbContextOptions passed should have the same lifetime specified in the AddDbContext call.

I can't make the database context a singleton otherwise concurrent calls to it would yield concurrency exceptions (due to the fact that the database context is not guaranteed to be thread safe).

Answer 1

A good way to use services inside of hosted services is to create a scope when needed. This allows to use services / db contexts etc. with the lifetime configuration they are set up with. Not creating a scope could in theory lead to creating singleton DbContexts and improper context reusing (EF Core 2.0 with DbContext pools).

To do this, inject an IServiceScopeFactory and use it to create a scope when needed. Then resolve any dependencies you need from this scope. This also allows you to register custom services as scoped dependencies should you want to move logic out of the hosted service and use the hosted service only to trigger some work (e.g. regularly trigger a task - this would regularly create scopes, create the task service in this scope which also gets a db context injected).

public class MyHostedService : IHostedService
{
    private readonly IServiceScopeFactory scopeFactory;

    public MyHostedService(IServiceScopeFactory scopeFactory)
    {
        this.scopeFactory = scopeFactory;
    }

    public void DoWork()
    {
        using (var scope = scopeFactory.CreateScope())
        {
            var dbContext = scope.ServiceProvider.GetRequiredService<MyDbContext>();
            …
        }
    }
    …
}