An ASP.NET MVC ValidationAttribute that ensures that an ABN (Australian Business Number) is both registered and Active.

AbnIsActiveAttribute.cs

public class AbnIsActiveAttribute : ValidationAttribute
{
	private static string AbnValidationUrl = "http://abr.business.gov.au/abrxmlsearch/AbrXmlSearch.asmx/ABRSearchByABN?searchString={0}&includeHistoricalDetails=N&authenticationGuid=74fd05cb-bbf6-4525-a68f-c553f6349222";
	private static string ShouldContainString = "<entityStatusCode>Active</entityStatusCode>";

	public AbnIsActiveAttribute()
	{
		ErrorMessage = "This ABN is inactive or invalid.";
	}

	public override bool IsValid(object value)
	{
		if (value == null)
			return true;

		var request = WebRequest.Create(string.Format(AbnValidationUrl, value.ToString().Replace(" ", "")));

		request.Timeout = 100000;
		request.ContentType = "text/xml; charset=utf-8";
		request.Proxy = WebRequest.GetSystemWebProxy();
		request.Method = "GET";

		return new StreamReader(request.GetResponse().GetResponseStream()).ReadToEnd().Contains(ShouldContainString);
	}

	protected override ValidationResult IsValid(object value, ValidationContext validationContext)
	{
		ErrorMessage = string.Format(ErrorMessage, validationContext.DisplayName);
		if (IsValid(value))
			return ValidationResult.Success;
		else
			return new ValidationResult(string.Format(ErrorMessage, validationContext.DisplayName));
	}
}