I am creating a custom dropdown validation just to check but it is not working and throwing an error?

private string _SelectedValue;

        public DropDownValidation(string selectedValue)
        {
            _SelectedValue = selectedValue;
        }

        public override ValidationResult IsValid(object value, ValidationContext validationContext)
        {

            if (value != null)
            {
                if (_SelectedValue == "----Select Country----")
                {
                    return new ValidationResult("Select Some Country Please.");
                }
                else if (_SelectedValue == "----Select State----")
                {
                    return new ValidationResult("Select Some State Please.");
                }
                else if (_SelectedValue == "----Select City----")
                {
                    return new ValidationResult("Select Some City Please");
                }
            }
            return ValidationResult.Success;
        }

Error	1	'ddl.Common.DropDownValidation.IsValid(object, System.ComponentModel.DataAnnotations.ValidationContext)': cannot change access modifiers when overriding 'protected' inherited member 'System.ComponentModel.DataAnnotations.ValidationAttribute.IsValid(object, System.ComponentModel.DataAnnotations.ValidationContext)'