microsoft-extensions-configuration

Validating .NET configuration options at startup is achieved by binding configuration sections to strongly-typed POCOs using ValidateOnStart, which enforces data annotations and custom rules to fail fast on misconfigurations before the application runs.

Complex validation rules for IOptions in ASP.NET Core are best implemented using the IValidateOptions<T> interface, enabling dependency-injection-friendly validators for cross-property, conditional, and dependency-aware configuration checks.

Configuring multiple named options instances allows you to register dedicated settings instances within your .NET application, each with its own specific validation rules applied through the Options pattern and IValidateOptions<T>.

Post-configuration with the .NET Options pattern should be used when you need to adjust default values after initial binding but before final validation, ensuring dynamically modified settings are correct before application startup completes.

ASP.NET Core options binding supports attribute-based Data Annotations validation, allowing you to decorate strongly-typed settings properties and enforce rules during startup via ValidateOnStart to prevent misconfigured applications from running.

Your .NET application crashes on startup when options validation fails because ValidateOnStart enforces fail-fast behavior, immediately halting execution if bound configuration settings violate data annotations or custom IValidateOptions<T> rules.