DocumentOrganizer/Diligent.WebAPI.Host/Middlewares/ModelValidationMiddleware.cs

using Diligent.WebAPI.Contracts.DTOs.Error;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;

namespace Diligent.WebAPI.Host.Middlewares
{
    [ExcludeFromCodeCoverage]
    public class ModelValidationMiddleware : IActionFilter
    {
        public void OnActionExecuted(ActionExecutedContext context) { }

        public void OnActionExecuting(ActionExecutingContext context)
        {
            if (!context.ModelState.IsValid)
            {
                // create response model
                var response = new ErrorResponseDto("Invalid model")
                {
                    // get model validation errors from ModelState
                    ValidationItems = context.ModelState
                    .Where(x => x.Value != null && x.Value.Errors.Count > 0)
                    .Select(x => new ValidationItemDto { Key = x.Key, Value = x.Value?.Errors.Select(e => e.ErrorMessage).First() })
                    .ToList()
                };
                // return validation model
                context.Result = new BadRequestObjectResult(response);
            }
        }
    }
}