Validation attribute
A validation attribute is metadata attached to a class or member that applies a rule for determining whether data satisfies specified requirements.
Building Web APIs with ASP.NET Core
A custom validation attribute is an application-defined attribute that implements a domain-specific input-validation rule.
- How it is used
- Guards client-supplied sort strings before a board-game API uses them in dynamic ordering.
- How deep it goes
- Works through an attribute subclass, its validation result, parameter placement, and OpenAPI filters.
- Examples
- Uses sort-direction and sort-column validators on a GET endpoint, then moves them into a request DTO.
- What you can do after
- Build it from scratch by subclassing ValidationAttribute, applying the rule to an input, and checking its error response.
Read how Building Web APIs with ASP.NET Core explains validation attribute →
ASP.NET Core in Action, 3e
A custom validation attribute is an application-defined attribute that implements a specialized validation rule, typically by deriving from ValidationAttribute.
- How it is used
- Extends property-level DataAnnotations validation with domain-specific rules in Razor Pages and MVC binding models.
- How deep it goes
- Explains derivation, IsValid, error results, reuse, and service lookup, while deferring complex cross-property business rules.
- Examples
- Uses a currency converter that accepts only GBP, USD, EUR, or CAD and rejects an invalid value such as £$%.
- What you can do after
- Build it from scratch by defining, applying, and testing a reusable ValidationAttribute for application-specific property rules.
Read how ASP.NET Core in Action, 3e explains validation attribute →
Pro ASP.NET Core 7, Tenth Edition
A custom validation attribute is an application-defined .NET attribute derived from ValidationAttribute that supplies validation logic for model data.
- How it is used
- Centralizes application-specific model rules so bound data receives consistent validation across controllers and Razor Pages.
- How deep it goes
- Works through property- and model-level IsValid implementations, service access, attribute placement, and a Razor Pages error-display workaround.
- Examples
- Uses a database-backed primary-key check and a class-level name-and-price rule on a product form.
- What you can do after
- Build it from scratch to replace duplicated controller and Razor Page checks with reusable property- or model-level validation.
Read how Pro ASP.NET Core 7, Tenth Edition explains validation attribute →
Related concepts
- Range attributeincludes · The Range attribute is a validation attribute that constrains a value to a specified minimum and maximum.
- RequiredAttributeincludes · RequiredAttribute is a .NET data-annotation validation attribute that requires an annotated property or field to have a supplied, non-null value.
- client-side validationcontrasts with · Client-side validation is checking user-supplied data in software running on the user's device before it is sent to a server or other remote system.
- IValidatableObjectalternative to · IValidatableObject is a .NET interface that lets an object perform validation involving its own values and relationships and return validation results.
- RegularExpression attributealternative to · RegularExpressionAttribute is a .NET validation attribute that accepts a value only when it matches a specified regular expression.
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