Getting DbContext when resolving a singleton

Question

Within ConfigureServices I have

services.AddDbContext<MyContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

as well as

services.AddSingleton<IMyModel>(s =>
{
    var dbContext = s.GetService<MyContext>();
    var lastItem= dbContext.Items.LastOrDefault();
    return new MyModel(lastItem);
});

But s.GetService<MyContext>() throws an error:

Cannot resolve scoped service 'MyContext' from root provider.

How can I achieve that? I don't want to inject MyDbContext in MyModel contructor as it is in a library which should have no reason to know about Entity Framework.

Kirk Larkin · Accepted Answer · 2019-11-08 12:37:20Z

16

AddDbContext defaults to using a scoped lifestyle:

Scoped lifetime services (AddScoped) are created once per client request (connection).

The reason an error is being thrown is that you're attempting to obtain an instance of MyContext from outside of a request. As the error message suggests, it is not possible to obtain a scoped service from the root IServiceProvider.

For your purposes, you can create a scope explicitly and use that for your dependency resolution, like so:

services.AddSingleton<IMyModel>(sp =>
{
    using (var scope = sp.CreateScope())
    {
        var dbContext = scope.ServiceProvider.GetService<MyContext>();
        var lastItem = dbContext.Items.LastOrDefault();
        return new MyModel(lastItem);
    }
});

This code above creates a scoped IServiceProvider that can be used for obtaining scoped services.

edited Nov 8, 2019 at 12:37

answered Sep 20, 2017 at 17:20

Kirk Larkin

94.4k17 gold badges235 silver badges214 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Simon Over a year ago

The line with new MyModel(lastItem) will only be called once when the server starts correct? So lastItem is loaded from the database when the server starts and will never change when during the server runtime new items are added to the DB

Kirk Larkin Over a year ago

@SimonHeinen Yeah, that's right. Technically, it'll happen when IMyModel is first requested, but it will be just the once and it won't change, as you've stated.

Collectives™ on Stack Overflow

Getting DbContext when resolving a singleton

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related