ValidatorFilter.cs

using System.Diagnostics;
using FluentValidation;
using Microsoft.AspNetCore.Mvc;

namespace MinimalApi.Filters;

public class ValidatorFilter<T>(IValidator<T> validator) : IEndpointFilter where T : class
{
    public async ValueTask<object?> InvokeAsync(EndpointFilterInvocationContext context, EndpointFilterDelegate next)
    {
        if (context.Arguments.FirstOrDefault(a => a?.GetType() == typeof(T)) is not T input)
        {
            return TypedResults.BadRequest();
        }

        var validationResult = await validator.ValidateAsync(input);

        if (validationResult.IsValid)
        {
            return await next(context);
        }

        var statusCode = StatusCodes.Status400BadRequest;
        var problemDetails = new ProblemDetails
        {
            Status = statusCode,
            Type = $"https://httpstatuses.io/{statusCode}",
            Title = "One or more validation errors occurred",
            Instance = context.HttpContext.Request.Path
        };

        problemDetails.Extensions["traceId"] = Activity.Current?.Id ?? context.HttpContext.TraceIdentifier;
        problemDetails.Extensions["errors"] = validationResult.ToDictionary();

        return TypedResults.Json(problemDetails, statusCode: StatusCodes.Status400BadRequest, contentType: "application/problem+json; charset=utf-8");
    }
}

public static class RouteHandlerBuilderExtensions
{
    public static RouteHandlerBuilder WithValidation<T>(this RouteHandlerBuilder builder) where T : class
        => builder.AddEndpointFilter<ValidatorFilter<T>>();
}