AspNetCore.Docs/aspnet/mvc/models/validation/sample/ClassicMovieAttribute.cs

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Globalization;
using Microsoft.AspNetCore.Mvc.ModelBinding.Validation;

namespace MVCMovie.Models
{
    public class ClassicMovieAttribute : ValidationAttribute, IClientModelValidator
    {
        private int _year;

        public ClassicMovieAttribute(int Year)
        {
            _year = Year;
        }

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            Movie movie = (Movie)validationContext.ObjectInstance;

            if (movie.Genre == Genre.Classic && movie.ReleaseDate.Year > _year)
            {
                return new ValidationResult(GetErrorMessage());
            }

            return ValidationResult.Success;
        }

        public void AddValidation(ClientModelValidationContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException(nameof(context));
            }

            MergeAttribute(context.Attributes, "data-val", "true");
            MergeAttribute(context.Attributes, "data-val-classicmovie", GetErrorMessage());

            var year = _year.ToString(CultureInfo.InvariantCulture);
            MergeAttribute(context.Attributes, "data-val-classicmovie-year", year);
        }

        private bool MergeAttribute(IDictionary<string, string> attributes, string key, string value)
        {
            if (attributes.ContainsKey(key))
            {
                return false;
            }

            attributes.Add(key, value);
            return true;
        }

        private string GetErrorMessage()
        {
            return "Classic movies must have a release year earlier than " + _year;
        }
    }
}