FluentValidation Study Portal

When validation is complex, needs async checks, localization, or cross-field logic. FluentValidation keeps rules in dedicated classes, making them testable and composable, whereas data annotations are limited to attribute-based, synchronous checks.

Use Include() to compose validators, RuleSet to toggle groups, or separate DTOs per use case. Avoid giant conditional validators—split contexts when rules diverge significantly.

Limit them to pure validation (checking invariants, referencing read-only dependencies). For workflows or state changes, push logic into application/domain services. Validators can query read models but shouldn’t mutate state or call external systems beyond existence checks.

It controls whether subsequent rules run after a failure. CascadeMode.Stop short-circuits to reduce noise and redundant work, which is useful for perf or to avoid duplicate messages.

Use RuleForEach(x => x.Items).SetValidator(new ItemValidator()); to apply nested validators per element, or RuleFor(x => x.Items).NotEmpty() for aggregate-level checks. Each nested validator has access to the child item context.

Use MustAsync or CustomAsync, inject the dependency (e.g., repository, API client), and ensure it supports cancellation tokens. Batch expensive checks to avoid N+1 calls.

Register validators in DI and add a pipeline behavior that resolves IValidator<TRequest>, executes them before the handler, and throws a ValidationException if failures exist. This keeps controllers thin and centralizes validation.

Provide fake implementations or mocks for the dependencies, instantiate the validator with them, and assert Validate results. Since validators are regular classes, tests run fast without ASP.NET hosting.

Use WithMessage(localizer["Key"]), configure ValidatorOptions.Global.LanguageManager, or override IStringSource to supply localized strings. Keep messages in resource files rather than hard-coding text.

Register validators with matching lifetimes (usually transient), inject scoped services via constructor, and avoid static validators. When using AddValidatorsFromAssemblyContaining, it defaults to transient, which honors DI scopes.