The back-story:

I'm running an Umbraco 7.2.1 site with Umbraco Forms 4.0.2 and I'm attempting to make my first custom field to attach to an existing form (created through the back-office).

The purpose of the control is to take entry of an email address and verify that the address domain is not on a blacklist.

I have a set of domains which are always blacklisted (temporary email providers for example) and the rest I wish to gather on a form-to-form basis by using a Forms Setting. The forever blacklisted domains are stored in a Config Setting and set in the constructor, the form-specific blacklisted domains are set in the back-office when the control is added.

When stepping through the Constructor I can see that the EmailAddressesToIgnore property is null, it then receives the value from the Config Setting and this value persists.

When stepping through the ValidateField method I can see that the value is set to the Config Setting just as it was after the constructor phase.

If I then go back into the back-office, the values set on the control are still present in the textbox, so somehow the values I set there are not available on the property.

Tl;dr:

Having added a simple text Setting to a custom FieldType, any data entered into this Setting is visible and stored in the back-office but not available when debugging the ValidateField method.

Have I made an incredibly simple mistake?

public class UniqueEmail : FieldType
{
    [Setting("Ignored email domains",
             description = "Enter email address domains which should be ignored, separate each with a comma. NOTE: You should only enter the value after the @ e.g. domain.co.uk",
             view = "uniqueemail")]
    public string EmailAddressesToIgnore { get; set; }

    public UniqueEmail()
    {
        //Provider
        Id = new Guid("D6A2C406-CF89-11DE-B075-55B055D89593 ");
        Name = "Unique Email Address";
        Description = "Renders a html input fieldKey"; //FieldType
        Icon = "icon-autofill";
        DataType = FieldDataType.String;
        SupportsRegex = true;

        // Pre-populate the field
        EmailAddressesToIgnore = string.IsNullOrWhiteSpace(EmailAddressesToIgnore)
            ? Properties.Settings.Default.Competitions_IgnoredEmailDomains.ToLower()
            : EmailAddressesToIgnore + "," + Properties.Settings.Default.Competitions_IgnoredEmailDomains.ToLower();
    }

    public override IEnumerable<string> ValidateField(Field field, IEnumerable<object> postedValues, HttpContextBase context)
    {
        // Perform the base validation first (lightest)
        postedValues = postedValues.ToList();
        var validationResults = base.ValidateField(field, postedValues, context).ToList();
        if (validationResults.Any()) return validationResults;

        // Perform ignored domain validation next (heavier)
        if (!string.IsNullOrWhiteSpace(EmailAddressesToIgnore))
        {
            validationResults.AddRange(from emailAddress in postedValues
                                       select emailAddress.ToString().ToLower().Split('@')
                                           into emailSections
                                           where emailSections.Count() == 2 &&
                                                 EmailAddressesToIgnore.SplitAndCleanCsvIntoList().Contains(emailSections[1])
                                           select "Email address cannot belong to an employee or temporary email service");

            if (validationResults.Any()) return validationResults;
        }

        return validationResults;
    }
}