数据注释

public class Dinner
    {
        public string ID { get; set; }
        public string Title { get; set; }
        public Category Category { get; set; }
        public DateTime? DateCreated { get; set; }
    }

该类(重要部分)的模型视图是

public class DinnerModelView
    {
        ...
        [UIHint("DatePicker")]
        [DateTime(ErrorMessage = "Invalida date")]
        public DateTime? DateCreated { get; set; }
    }

其中datetimeattribute是

public class DateTimeAttribute : ValidationAttribute
    {
        public DateTimeAttribute () : base (() => "Invalid date") { }
        public DateTimeAttribute(string errorMessage) : base(() => errorMessage) { } 
        public override bool IsValid(object value)
        {
            if (value == null)
                return true;

            bool isValid = false;
            if (value is DateTime)
                isValid = true;

            DateTime tmp;
            if (value is String)
            {
                if(String.IsNullOrEmpty((string)value))
                    isValid = true;
                else
                    isValid = DateTime.TryParse((string)value, out tmp);
            }

            return isValid;
        }
    }

但是,模型状态错误仍然表示“值'XXXX'对于datecreated无效。” 我无法替换此消息。为什么?