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What is the 'captive dependency' problem, and how does the DI container help catch it? What are keyed services (introduced in .NET 8), and what problem do they solve compared to multiple unnamed registrations? How do you use IServiceScopeFactory to manually create and manage a DI scope outside the normal request pipeline? How does the DI container's disposal behavior work for IDisposable and IAsyncDisposable services across different lifetimes? How would you unit test a service that depends on multiple injected interfaces without spinning up the full DI container?
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What is the Options pattern's relationship to DI, and how does IOptions<T> itself get resolved through the container?

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Answer

IOptions<T>, IOptionsSnapshot<T>, and IOptionsMonitor<T> are themselves services resolved through the DI container (registered automatically when you call services.AddOptions() or implicitly via Configure<T>()), demonstrating that the Options pattern is built entirely on top of the standard DI infrastructure rather than being a separate mechanism -- IOptions<T> is registered as Singleton internally, IOptionsSnapshot<T> as Scoped, explaining their respective behavior around configuration change visibility.
// Configure<T> implicitly registers the necessary IOptions<T> infrastructure
builder.Services.Configure<SmtpSettings>(builder.Configuration.GetSection("Smtp"));

// Under the hood, this makes IOptions<SmtpSettings>, IOptionsSnapshot<SmtpSettings>,
// and IOptionsMonitor<SmtpSettings> all resolvable via the same DI container
Real-world example Understanding that IOptionsSnapshot<T> is registered as Scoped explains precisely why it can't be safely injected into a Singleton service -- it's the exact same captive dependency problem as any other Scoped service being consumed by a Singleton.

Common follow-ups: Why does IOptionsMonitor<T> avoid the captive dependency problem despite being usable in Singletons?;How would you build a custom pattern following these same DI principles?

Configuration & Options;Background Services
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