Validation

Validation with DataAnnotations

DataAnnotations attributes on TRequest are validated automatically before HandleAsync is called. On failure, a 400 Bad Request response is returned in the standard RFC 9457 problem details format.

public class CreateUserRequest
{
    [Required]
    [MaxLength(100)]
    public required string Name { get; init; }

    [Required]
    [EmailAddress]
    public required string Email { get; init; }
}

A failed request returns a response in the following shape:

{
  "type": "https://tools.ietf.org/html/rfc9110#section-15.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "errors": {
    "Email": ["The Email field is not a valid e-mail address."]
  }
}

Disabling automatic validation

To opt out of automatic validation globally, pass options to AddAxisEndpoints:

builder.Services.AddAxisEndpoints(options =>
{
    options.DisableDataAnnotationsValidation = true;
});

Validation with FluentValidation

For validation logic that DataAnnotations cannot express, FluentValidation can be integrated via a custom IEndpointFilter. AxisEndpoints does not provide a dedicated FluentValidation package because the filter approach is straightforward and keeps the dependency opt-in.

// A reusable filter that resolves IValidator<TRequest> from DI.
public class FluentValidationFilter<TRequest> : IEndpointFilter
{
    private readonly IValidator<TRequest> _validator;

    public FluentValidationFilter(IValidator<TRequest> validator) => _validator = validator;

    public async ValueTask<object?> InvokeAsync(
        EndpointFilterInvocationContext context,
        EndpointFilterDelegate next)
    {
        var request = context.Arguments.OfType<TRequest>().FirstOrDefault();

        if (request is not null)
        {
            var result = await _validator.ValidateAsync(request);

            if (!result.IsValid)
            {
                var errors = result.Errors
                    .GroupBy(e => e.PropertyName)
                    .ToDictionary(g => g.Key, g => g.Select(e => e.ErrorMessage).ToArray());

                return TypedResults.ValidationProblem(errors);
            }
        }

        return await next(context);
    }
}

// Register the validator and apply the filter to the endpoint.
builder.Services.AddScoped<IValidator<CreateUserRequest>, CreateUserRequestValidator>();

public class CreateUserEndpoint : IEndpoint<CreateUserRequest, Response<CreateUserResponse>>
{
    public void Configure(IEndpointConfiguration config)
    {
        config.Post("/users")
            .Tags("Users")
            .AddFilter<FluentValidationFilter<CreateUserRequest>>();
    }
    // ...
}

For more on custom filters, see the Filters guide. For CSV row-level validation, see Row Validation.