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How does the ASP.NET Core configuration system layer multiple sources (appsettings.json, environment variables, command line) together? What does GetSection and GetValue do differently when reading configuration values in ASP.NET Core? What is the Options pattern, and why is it preferred over directly injecting IConfiguration into services? How does appsettings.{Environment}.json layering work, and how is the current environment determined? How do environment variables map to nested configuration keys in ASP.NET Core, given environment variables can't contain colons on some platforms? What is user secrets (dotnet user-secrets), and why should it only be used for local development, never production? How does the command-line configuration provider let you override settings via arguments when running an application? How does the order of configuration provider registration affect which value wins when the same key is defined in multiple sources? What is the difference between IOptions<T>, IOptionsSnapshot<T>, and IOptionsMonitor<T>? How does the configuration binder handle nested objects, arrays, and complex types when binding a JSON section to a C# class?
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How do you validate configuration options at application startup to fail fast on misconfiguration?

Intermediate

Answer

Using ValidateDataAnnotations() combined with ValidateOnStart() on the options builder ensures configuration is validated immediately when the application starts (rather than lazily on first use), causing the application to fail to launch with a clear error message if required settings are missing or invalid -- far preferable to a confusing NullReferenceException or incorrect behavior discovered only when a specific code path first accesses the misconfigured option deep into runtime.
public class SmtpSettings {
    [Required] public string Host { get; set; } = "";
    [Range(1, 65535)] public int Port { get; set; }
}

builder.Services.AddOptions<SmtpSettings>()
    .Bind(builder.Configuration.GetSection("Smtp"))
    .ValidateDataAnnotations()
    .ValidateOnStart();
Real-world example A misconfigured production deployment missing the required Smtp:Host setting now fails immediately at startup with a clear 'DataAnnotation validation failed' error, instead of silently running until the first email-send attempt fails hours later with a confusing null reference exception.

Common follow-ups: How do you write custom validation logic beyond DataAnnotations using IValidateOptions?;What happens without ValidateOnStart -- when does validation actually run?

Diagnostics & Performance;Global Exception Handling & Middleware
All 15 Configuration & Options questions