What goes into DbContextOptions when invoking a new DbContext?

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If you really want to create the context manually, then you can configure it like this:

var optionsBuilder = new DbContextOptionsBuilder<ApplicationDbContext>();
optionsBuilder.UseSqlServer(Configuration.GetConnectionStringSecureValue("DefaultConnection"));
_context = new ApplicationDbContext(optionsBuilder.Options);

(The DbContextOptionsBuilder<ApplicationDbContext> class is the type of options argument in services.AddDbContext<ApplicationDbContext>(options =>).
But in the controller, you don’t have access to Configuration object, so you would have to expose it as a static field in Startup.cs or use some other trick, which is all bad practice.

The best way to obtain ApplicationDbContext is to get it through DI:

public GigsController(ApplicationDbContext context)
{
    _context = context;
}

The DI container will take care of instantiating and disposing of ApplicationDbContext. Note that you have everything correctly configured in Startup.cs:

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

services.AddIdentity<ApplicationUser, IdentityRole>()
        .AddEntityFrameworkStores<ApplicationDbContext>()
        .AddDefaultTokenProviders();

That’s configuring DI, so why not just use it?

One more note about the default constructor of DbContext: In EF6 it was done like this: public ApplicationDbContext(): base("DefaultConnection") {}. Then the base object would use System.Configuration.ConfigurationManager static class to obtain the connection string named DefaultConnection from web.config. The new Asp.net Core and EF Core is designed to be as much decoupled as possible, so it should not take dependencies on any configuration system. Instead, you just pass a DbContextOptions object – creating that object and configuring it is a separate concern.

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