Quick Answer
Model validation checks that bound model properties satisfy their data annotation constraints ([Required], [MaxLength], [EmailAddress], [Range]). After binding, ASP.NET Core populates ModelState — if invalid, ModelState.IsValid is false and you return a 400 response. FluentValidation is a popular alternative to data annotations for complex rules.
Answer
Model validation runs immediately after model binding and before the action method executes.
Validation rules are typically expressed via data annotations (e.g., [Required], [Range]).
Errors are added to ModelState.
Controllers or Razor Pages check ModelState.IsValid to decide whether to continue or return validation errors.
Validation can also be implemented via IValidatableObject or custom validators, ensuring that only valid data reaches business logic.
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Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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