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.

Answer 1

AddDbContext defaults to using a scoped lifestyle:

Scoped lifetime services are created once per request.

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.