Data Annotation valuecontext始终为null

Question

public class ExcludeChar : ValidationAttribute
    {
        string _characters;
        public ExcludeChar(string characters)
        {
            _characters = characters;
        }
        protected override ValidationResult IsValid(object value, System.ComponentModel.DataAnnotations.ValidationContext validationContext)
        {
            if (value != null)
            {
                for (int i = 0; i < _characters.Length; i++)
                {
                    var valueAsString = value.ToString();
                    if (valueAsString.Contains(_characters[i]))
                    {
                        var errorMessage = FormatErrorMessage(validationContext.DisplayName);
                        return new ValidationResult(errorMessage);
                    }
                }
            }
            return ValidationResult.Success;
        }
    }

[ExcludeChar("@#$",ErrorMessage = "Please select a option.")]
        public string LSESelected1
        {
}

我在没有validationContext的情况下调用ViewModel类中的一个验证，验证工作正常，如果我检查validationContext它总是显示“我为空”。