---

name: aspnet-validation description: | ASP.NET Core validation with FluentValidation, Data Annotations, and model validation. Covers custom validators and validation pipeline.

USE WHEN: user mentions "FluentValidation", "Data Annotations", "model validation", ".NET validation", "input validation", "validator", "validation pipeline"

DO NOT USE FOR: Spring Validation - use spring-validation, Zod/Yup - use frontend validation skills allowed-tools: Read, Grep, Glob, Write, Edit

ASP.NET Validation - Quick Reference

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: aspnet-core for validation documentation.

Data Annotations

public class CreateUserRequest
{
    [Required(ErrorMessage = "Name is required")]
    [StringLength(100, MinimumLength = 2)]
    public string Name { get; set; } = default!;

    [Required]
    [EmailAddress]
    [StringLength(255)]
    public string Email { get; set; } = default!;

    [Required]
    [Range(0, 150)]
    public int Age { get; set; }

    [Url]
    public string? Website { get; set; }
}

FluentValidation Setup

// Install: dotnet add package FluentValidation.AspNetCore
builder.Services.AddValidatorsFromAssemblyContaining<Program>();

FluentValidation Patterns

public class CreateUserValidator : AbstractValidator<CreateUserRequest>
{
    private readonly IUserRepository _repository;

    public CreateUserValidator(IUserRepository repository)
    {
        _repository = repository;

        RuleFor(x => x.Name)
            .NotEmpty().WithMessage("Name is required")
            .Length(2, 100);

        RuleFor(x => x.Email)
            .NotEmpty()
            .EmailAddress()
            .MaximumLength(255)
            .MustAsync(BeUniqueEmail).WithMessage("Email already exists");

        RuleFor(x => x.Age)
            .InclusiveBetween(0, 150);

        RuleFor(x => x.Password)
            .NotEmpty()
            .MinimumLength(12)
            .Matches(@"[A-Z]").WithMessage("Must contain uppercase letter")
            .Matches(@"[a-z]").WithMessage("Must contain lowercase letter")
            .Matches(@"\d").WithMessage("Must contain digit")
            .Matches(@"[@$!%*?&]").WithMessage("Must contain special character");
    }

    private async Task<bool> BeUniqueEmail(string email, CancellationToken ct)
    {
        return !await _repository.ExistsAsync(u => u.Email == email);
    }
}

Validation Filter (Minimal API)

public class ValidationFilter<T> : IEndpointFilter where T : class
{
    public async ValueTask<object?> InvokeAsync(
        EndpointFilterInvocationContext context,
        EndpointFilterDelegate next)
    {
        var validator = context.HttpContext.RequestServices.GetService<IValidator<T>>();
        var argument = context.Arguments.OfType<T>().FirstOrDefault();

        if (validator is not null && argument is not null)
        {
            var result = await validator.ValidateAsync(argument);
            if (!result.IsValid)
                return Results.ValidationProblem(result.ToDictionary());
        }

        return await next(context);
    }
}

Collection and Nested Validation

public class OrderValidator : AbstractValidator<CreateOrderRequest>
{
    public OrderValidator()
    {
        RuleFor(x => x.Items)
            .NotEmpty().WithMessage("Order must have at least one item");

        RuleForEach(x => x.Items).ChildRules(item =>
        {
            item.RuleFor(x => x.ProductId).GreaterThan(0);
            item.RuleFor(x => x.Quantity).InclusiveBetween(1, 1000);
        });
    }
}

Anti-Patterns

Anti-Pattern	Why It's Bad	Correct Approach
Validation in controllers	Repetitive, inconsistent	Use FluentValidation + filter
Only client-side validation	Easily bypassed	Always validate server-side
No async validation	Can't check DB uniqueness	Use MustAsync
Generic error messages	Bad UX	Provide specific .WithMessage()

Quick Troubleshooting

Issue	Likely Cause	Solution
Validator not running	Not registered	Call AddValidatorsFromAssembly
Async validator timeout	DB query slow	Add appropriate indexes
ModelState not checked	Missing [ApiController]	Add attribute or check manually
Nested validation skipped	Missing SetValidator	Use ChildRules or SetValidator