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FluentValidation · Quick recall Q&A

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Mid-level6 min read
Rapid overview

Quick recall Q&A

Q: When should you reach for FluentValidation over data annotations? A: 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.
Q: How do you share rules between create and update flows? A: 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.
Q: How do you keep validators from doing business logic? A: 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.
Q: What’s the role of CascadeMode? A: 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.
Q: How do you validate collections? A: 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.
Q: How do you handle async validators hitting external services? A: 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.
Q: How do you integrate FluentValidation with MediatR pipelines? A: 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.
Q: How do you test validators that depend on services? A: 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.
Q: How can you customize error messages for localization? A: 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.
Q: How do you prevent validators from capturing scoped services incorrectly? A: 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.

See also