How to handle DBContext instancing in Entity Framework Core

Question

Coming from a Java environment I'm having a few issues wrapping my head about the inheritable DBContext class. with Java EE I used to:

1. Set up one or multiple DB Contexts (if separate clusters of entities where affected);
2. Added the Context to respective DataAccessor classes that handled the query execution via DI;

Now with EF Core practically all samples I see create an instance of a MyDBContext by calling the default constructor:

 using (var db = new myContext()){...}

This raises a few questions for me:

• With this Method each DataAccessor class that calls the constructor has its own instance of the Context. Wouldn't it be nicer to only ave one and use DI to inject it when needed?
• How do I call the constructor if i didn't overload OnConfiguring(...) to pass the options, but instead used AddDbContext as a Service in Startup.cs? Now the overloaded constructor with the options expects them to be passed on each time the constructor is called.
• Is having multiple DBContexts per application/Db even a good practise with EF Core?

Answer 1

Normally you would want single instance per request scope

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<ApplicationDbContext>(options =>
        // use options as you would use them in the .OnConfiguring
        options.UseSqlServer("your connectionString");
    );
}

If you use constructor injection in services, ASP.NET service provider will resolve db context as constructor parameter. All services that have db context this way will share same instance.

public class ServiceDependentOnContext
{
    private readonly ApplicationDbContext dbContext;

    public ServiceDependentOnContext(ApplicationDbContext dbContext)
    {
        this.dbContext = dbContext;
    }
}

Make sure you configure your service for dependency injection as well

// This method gets called by the runtime. Use this method to add services to the container.

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<ApplicationDbContext>(options =>
        options.UseSqlServer("your connectionString")
    );

    services.AddScoped<ServiceDependentOnContext>();
}