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[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public sealed class NotEqualToAttribute : ValidationAttribute
{
    private const string DefaultErrorMessage = "{0} cannot be the same as {1}.";

    public string OtherProperty { get; private set; }

    public NotEqualToAttribute(string otherProperty)
        : base(DefaultErrorMessage)
    {
        if (string.IsNullOrEmpty(otherProperty))
        {
            throw new ArgumentNullException("otherProperty");
        }

        OtherProperty = otherProperty;
    }

    public override string FormatErrorMessage(string name)
    {
        return string.Format(ErrorMessageString, name, OtherProperty);
    }

    protected override ValidationResult IsValid(object value,
                          ValidationContext validationContext)
    {
        if (value != null)
        {
            var otherProperty = validationContext.ObjectInstance.GetType()
                               .GetProperty(OtherProperty);

            var otherPropertyValue = otherProperty
                          .GetValue(validationContext.ObjectInstance, null);

            if (value.Equals(otherPropertyValue))
            {
                return new ValidationResult(
                  FormatErrorMessage(validationContext.DisplayName));
            }
        }

        return ValidationResult.Success;
    }
}