Singleton Scope for EF's DbContext

The lifetime of some services including DbContext can be configured this way:

services.AddDbContext<ApplicationDbContext>(
    options => { options.UseSqlServer("YourConnectionString"); },
    ServiceLifetime.Singleton);

REF

Singleton-scope is a very bad idea for your context. Request-scope is what you should be using, as it's essentially a singleton for the life of the request.

As to why you're getting errors when using request-scope, I can't say for sure. Assuming that the entities you're utilizing all originate from the same context type, and that you're properly injecting the context everywhere it's needed, there should never be multiple context instances in play.

EDIT

After re-reading your question, it sounds as if your services are actually initializing the context in their constructors or something. If that's the case, that's your problem. You context should be injected into your services, i.e.:

public class AccountService : IAccountService
{
    protected readonly DbContext context;

    public AccountService(DbContext context)
    {
        this.context = context;
    }

    ...
}

Then, Ninject will properly inject the request-scoped instance of MyApplicationContext when newing up any of the services.

Dan, you are creating a bottleneck when you scope a single DBContext for the entire application. Underneath the hood, Entity Framework will handle how many objects you need rather efficiently. If you go deeper into internals, the actual objects contacting the database do the same thing. So your attempt to optimize by making a singleton may actually be creating a very big problem.