nullable integer validation

NullableIntegerAttribute.cs

using System.ComponentModel.DataAnnotations;

namespace Project.MvvmFramework.ValidationAttributes
{
    public class NullableIntegerAttribute : ValidationAttribute
    {
        private readonly int _maxDigits;
        private readonly string _propertyName;
        const string MessageFormat = "{0} can only contain up to {1} numeric characters.";

        public NullableIntegerAttribute(int maxDigits, string propertyName)
        {
            _maxDigits = maxDigits;
            _propertyName = propertyName;
        }

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            var propertyInfo = validationContext.ObjectType.GetProperty(_propertyName);

            var retrievedValue = propertyInfo.GetValue(validationContext.ObjectInstance, null) as string;

            long parsed;
            if (string.IsNullOrEmpty(retrievedValue) || (retrievedValue.Length <= _maxDigits && long.TryParse(retrievedValue, out parsed)))
            {
                return ValidationResult.Success;
            }
            return new ValidationResult(string.Format(MessageFormat, _propertyName, _maxDigits));
        }
    }
}