IdentityShroud/IdentityShroud.Api/Validation/ValidateFilter.cs

using FluentValidation;

namespace IdentityShroud.Api;

public class ValidateFilter<T> : IEndpointFilter where T : class
{
    public async ValueTask<object?> InvokeAsync(
        EndpointFilterInvocationContext context,
        EndpointFilterDelegate next)
    {
        // Grab the deserialized argument (the DTO) from the context.
        // The order of arguments matches the order of parameters in the endpoint delegate.
        var dto = context.Arguments
            .FirstOrDefault(arg => arg is T) as T;

        if (dto == null)
            return Results.BadRequest("Unable to read request body.");

        // Resolve the matching validator from DI.
        var validator = context.HttpContext.RequestServices
            .GetService<IValidator<T>>();

        if (validator != null)
        {
            var validationResult = await validator.ValidateAsync(dto);
            if (!validationResult.IsValid)
                return Results.ValidationProblem(validationResult.ToDictionary());
        }

        // Validation passed – continue to the actual handler.
        return await next(context);
    }
}