All 15 Configuration & Options Pattern questions
More in Configuration & Options Pattern
What is user secrets (dotnet user-secrets), and why should it be used only for local development, never production? How do you use IHostEnvironment/IWebHostEnvironment to conditionally configure services based on the current environment? How does the command-line configuration provider let you override settings via arguments when running an application, and what's its precedence? What is the difference between IOptions<T>, IOptionsSnapshot<T>, and IOptionsMonitor<T>, and which is safe to inject into a singleton? How does configuration hot-reload work with reloadOnChange for appsettings.json, and what are its limitations? How does the configuration binder handle nested objects and arrays when binding a JSON section to a strongly-typed C# class? How would you write a custom configuration provider to load settings from an unsupported source, like a remote configuration service or database? How would you design configuration for a multi-tenant ASP.NET Core application where each tenant needs different settings resolved at runtime? How do you write a custom IValidateOptions<T> implementation for validation logic beyond what DataAnnotations attributes can express?
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ASP.NET Core › Configuration & Options Pattern

How do you validate configuration options at application startup to fail fast on misconfiguration, using ValidateDataAnnotations and ValidateOnStart?

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Answer

Chaining .ValidateDataAnnotations().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 for missing or invalid required settings, instead of a confusing failure 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 validation error, instead of 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?

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