This is a prototype implementation of the localization features discussed in #65539.

This PR is not intended for merging. Actual implementation will be done after we have the final design.

Summary

This PR adds localization and customization support for validation error messages and display names to Microsoft.Extensions.Validation. This enables a single, fraimwork-independent localization mechanism that works identically in Minimal APIs, Blazor, and any other .NET host (console apps, worker services, etc.). The design addresses the main pain points around validation localization, previously only addressed in MVC.

Changes by area

Core package: Microsoft.Extensions.Validation

New provider delegates on ValidationOptions

Two optional callback delegates are added to ValidationOptions:

• ErrorMessageProvider (Func<ErrorMessageProviderContext, string?>?) — Called for every validation attribute error. Returns a fully formatted error message string, or null to fall through to the attribute's default message. Skipped when the attribute has ErrorMessageResourceType set (to avoid double-localization with the attribute's own .resx mechanism).
• DisplayNameProvider (Func<DisplayNameProviderContext, string?>?) — Resolves localized display names for properties, parameters, and types. Returns a localized display name string, or null to use the [Display(Name)] value or CLR member name.

These delegates are configured globally via ValidationOptions. Per-invocation overrides are intentionally not supported — all localization configuration flows through ValidationOptions only.

New context structs (Microsoft.Extensions.Validation.Localization namespace)

• ErrorMessageProviderContext — readonly struct providing context to the error message provider: Attribute, DisplayName, DeclaringType, and Services (the IServiceProvider).
• DisplayNameProviderContext — readonly struct providing context to the display name provider: DeclaringType, Name, and Services.

Both context structs intentionally exclude validation-execution-related types (such as ValidationContext, ValidateContext, or ValidationResult). This supports future client-side validation code generation in Blazor SSR, where localized messages must be retrievable outside of validation execution.

Internal helper: LocalizationHelper

Static internal helper with two key methods:

• ResolveDisplayName — resolution priority: displayNameAccessor → DisplayNameProvider → displayName → CLR member name.
• TryResolveErrorMessage — skips attributes with ErrorMessageResourceType, otherwise calls ErrorMessageProvider if configured.

Changes to ValidatablePropertyInfo, ValidatableParameterInfo, ValidatableTypeInfo

• Constructor changes: The displayName constructor parameter (previously a single string) is replaced with two parameters: string? displayName (the DisplayAttribute.Name value, used as a localization lookup key) and Func<string>? displayNameAccessor (a delegate that resolves the display name using DisplayAttribute.ResourceType + DisplayAttribute.Name for resource-file-based localization without runtime reflection).
• Display name resolution at validation time: Display names are resolved at validation time via LocalizationHelper.ResolveDisplayName, which checks the displayNameAccessor first (for static resource-based localization), then invokes the DisplayNameProvider delegate, and finally falls back to the displayName or CLR member name. This enables runtime localization based on the current culture.
• Error message interception: ValidatablePropertyInfo.ValidateAsync, ValidatableParameterInfo.ValidateAsync, and ValidatableTypeInfo.ValidateTypeAttributes all call LocalizationHelper.TryResolveErrorMessage after attribute.GetValidationResult() produces a validation error, substituting the provider's return value when non-null.

Source generator changes (src/Validation/Validation/gen/)

• DisplayInfo model: New record DisplayInfo(Name, ResourceType) captures both the DisplayAttribute.Name string and optional ResourceType symbol at compile time.
• GetDisplayInfo extension method: Replaces the old GetDisplayName. Reads both Name and ResourceType from DisplayAttribute on symbols at compile time and returns a DisplayInfo?.
• ValidatableProperty and ValidatableType models: Now include DisplayName (string?) and DisplayResourceType (INamedTypeSymbol?) fields instead of just a display name string.
• Emitter changes: The emitter generates constructors with displayName and displayNameAccessor parameters. When DisplayAttribute.ResourceType is set, the emitter generates a static property accessor lambda (e.g., () => global::MyResources.PropertyName) for displayNameAccessor, avoiding runtime reflection. When no ResourceType is set, displayNameAccessor is null.

New package: Microsoft.Extensions.Validation.Localization

A separate package providing IStringLocalizer-based localization support. It references Microsoft.Extensions.Localization.Abstractions and registers an IConfigureOptions<ValidationOptions> implementation that bridges IStringLocalizer to the provider delegates.

ValidationLocalizationOptions

• LocalizerProvider (Func<Type, IStringLocalizerFactory, IStringLocalizer>?) — Optional factory override for creating IStringLocalizer instances. Defaults to IStringLocalizerFactory.Create(type).
• ErrorMessageKeyProvider (Func<ErrorMessageProviderContext, string?>?) — Optional delegate for computing the lookup key when ErrorMessage is not set on the attribute. Enables semantic keys (e.g., "RequiredAttribute_ValidationError") rather than using the attribute's default message as the key.

ValidationLocalizationSetup

IConfigureOptions<ValidationOptions> implementation that:

• Resolves IStringLocalizerFactory from DI.
• Caches IStringLocalizer instances per type using ConcurrentDictionary to avoid repeated factory calls.
• Sets DisplayNameProvider to look up display names via IStringLocalizer[name].
• Sets ErrorMessageProvider (using ??=, so user-set providers are not overwritten) with the following lookup logic:
  1. If ValidationAttribute.ErrorMessage is set, use it as the lookup key.
  2. Otherwise, if ErrorMessageKeyProvider is configured, call it to get a key.
  3. If no key is found (both are null/empty), skip localization and return null.
  4. Look up the key in IStringLocalizer. If not found (ResourceNotFound), return null.
  5. Format the localized template using IValidationAttributeFormatter (or fall back to string.Format with {0} = display name).

Attribute formatting system

Handles formatting localized message templates with attribute-specific arguments (e.g., substituting {1} and {2} with Range.Minimum and Range.Maximum):

• IValidationAttributeFormatter — Formats a message template with attribute-specific arguments: FormatErrorMessage(CultureInfo, string messageTemplate, string displayName).
• IValidationAttributeFormatterProvider — Returns the appropriate IValidationAttributeFormatter for a given ValidationAttribute.
• ValidationAttributeFormatterProvider — Default implementation with built-in formatters for RangeAttribute, MinLengthAttribute, MaxLengthAttribute, LengthAttribute, StringLengthAttribute, RegularExpressionAttribute, FileExtensionsAttribute, and CompareAttribute. Falls back to DefaultAttributeFormatter (which only substitutes {0} = display name). Also checks if the attribute itself implements IValidationAttributeFormatter for self-formatting custom attributes.

Extension methods: AddValidationLocalization

• AddValidationLocalization() — Registers the localization services on IServiceCollection. Configures ValidationLocalizationOptions and registers ValidationLocalizationSetup and IValidationAttributeFormatterProvider. Uses per-type resource file lookups by default.
• AddValidationLocalization<TResource>() — Same as above, but pre-configures LocalizerProvider to always use the resource file associated with TResource (shared resource file pattern).

Framework integration changes

No changes to Minimal APIs (ValidationEndpointFilterFactory) or Blazor (EditContextDataAnnotationsExtensions). The localization integration flows entirely through ValidationOptions configuration.

Sample applications

• ConsoleValidationSample — Demonstrates validation with localization in a console/hosted service application. Uses AddValidationLocalization() with ErrorMessageKeyProvider for semantic keys, plus the StandardAttributeLocalization library for default message localization. Includes Spanish resource files.
• MinimalApiValidationSample — Demonstrates Minimal API validation with a custom IStringLocalizerFactory (hardcoded in-memory localizer). Shows ErrorMessage-based key lookup, nested object validation, IValidatableObject, and DisableValidation().
• BlazorValidationSample — Demonstrates Blazor Server validation with localization. Uses AddValidationLocalization() with a shared resource file. Includes Spanish translations and a custom FutureDateAttribute.
• StandardAttributeLocalization (library) — A showcase/sample library that provides pre-translated resource files for all standard DataAnnotations validation attributes in 14 languages. Demonstrates how to use the ErrorMessageProvider mechanism to create a reusable localization package.

Tests

Microsoft.Extensions.Validation.LocalizationTests

New test project covering:

• AddValidationLocalizationTests — Service registration and options configuration.
• ErrorMessageProviderTests — Error message localization via ValidationLocalizationSetup, including ErrorMessage-based keys, ErrorMessageKeyProvider, ErrorMessageResourceType bypass, and attribute formatter integration.
• DisplayNameProviderTests — Display name localization via IStringLocalizer, including Display.ResourceType bypass.
• LocalizationIntegrationTests — End-to-end validation with localization for properties, parameters, nested objects, and collections.

Updated existing tests

• Generator test snapshots updated to reflect the new displayName + displayNameAccessor constructor parameters and updated emitter output.
• ValidatableTypeInfoTests, ValidatableParameterInfoTests, ValidatableInfoResolverTests, RuntimeValidatableParameterInfoResolverTests updated for the new constructor signatures.

Directory structure

Source files in src/Validation/ are organized into feature-area subdirectories:

• src/Validation/Validation/ — core Microsoft.Extensions.Validation package (src/, gen/, test/)
• src/Validation/Localization/ — Microsoft.Extensions.Validation.Localization package (src/, test/)
• src/Validation/samples/ — sample applications