ASP.NET Core PRG TempData Helpers

ModelStateHelper.cs

public class ModelStateTransferValue
{
    public string Key { get; set; }
    public string AttemptedValue { get; set; }
    public object RawValue { get; set; }
    public ICollection<string> ErrorMessages { get; set; } = new List<string>();
}

public static class ModelStateHelper
{
    public static string SerialiseModelState(ModelStateDictionary modelState)
    {
        var errorList = modelState.Select(kvp => new ModelStateTransferValue
        {
            Key = kvp.Key,
            AttemptedValue = kvp.Value.AttemptedValue,
            RawValue = kvp.Value.RawValue,
            ErrorMessages = kvp.Value.Errors.Select(err => err.ErrorMessage).ToList(),
        });

        return JsonConvert.SerializeObject(errorList);
    }

    public static ModelStateDictionary DeserialiseModelState(string serialisedErrorList)
    {
        var errorList = JsonConvert.DeserializeObject<List<ModelStateTransferValue>>(serialisedErrorList);

        var modelState = new ModelStateDictionary();

        foreach (var item in errorList)
        {
            modelState.SetModelValue(item.Key, item.RawValue, item.AttemptedValue);

            foreach (var error in item.ErrorMessages)
            {
                modelState.AddModelError(item.Key, error);
            }
        }

        return modelState;
    }
}

ModelStateTransfer.cs

public abstract class ModelStateTransfer : ActionFilterAttribute
{
    protected const string Key = nameof(ModelStateTransfer);
}

public class ExportModelStateAttribute : ModelStateTransfer
{
    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        //Somente exporta se o ModelState nao for valido
        if (!filterContext.ModelState.IsValid)
        {
            if (filterContext.HttpContext.IsAjaxRequest())
                ProcessAjax(filterContext);
            else ProcessNormal(filterContext);
        }

        base.OnActionExecuted(filterContext);
    }

    public void ProcessNormal(ActionExecutedContext filterContext)
    {
        //Exporta se estiver redirecionando
        if (filterContext.Result is RedirectResult || filterContext.Result is RedirectToRouteResult
                || filterContext.Result is RedirectToActionResult)
        {
            var controller = filterContext.Controller as Controller;

            if (controller != null && filterContext.ModelState != null)
            {
                var modelState = ModelStateHelper.SerialiseModelState(filterContext.ModelState);
                controller.TempData[Key] = modelState;
            }
         }
    }

    public void ProcessAjax(ActionExecutedContext filterContext)
    {
        var controller = filterContext.Controller as Controller;

        if (controller != null && filterContext.ModelState != null)
        {
            var modelState = ModelStateHelper.SerialiseModelState(filterContext.ModelState);
            filterContext.Result = new JsonResult(modelState);
        }
    }
}

public class ImportModelStateAttribute : ModelStateTransfer
{
    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        var controller = filterContext.Controller as Controller;
        var serialisedModelState = controller?.TempData[Key] as string;

        if (serialisedModelState != null)
        {
            //Somente importa se estivermos visualizando
            if (filterContext.Result is ViewResult)
            {
                var modelState = ModelStateHelper.DeserialiseModelState(serialisedModelState);
                filterContext.ModelState.Merge(modelState);
            }
            else
            {
                //Caso contrario, remove-o
                controller.TempData.Remove(Key);
            }
        }

        base.OnActionExecuted(filterContext);
    }
}

@thiagomajesk coloca o codigo de como esta sendo chamado no controller?

Tried to run it with checkboxes? Will explode with following error InvalidOperationException: The parameter conversion from type 'Newtonsoft.Json.Linq.JArray' to type 'System.Boolean' failed because no type converter can convert between these types.