MVC, Model, Index DataAnnotation issue.

[BrailleSchool]

Hi, guys,

I have a problem that I have not been able to figure out.
In some of my MSSQL columns, I am requiring that the information inserted is unique. Such as ITIN or SSN, FEIN etc.
The values are string, so they are automatically nullable based on my research.
I am using the [Index] DataAnnotation on these columns to ensure that the values inserted are unique.

However, my problem is that because I have the [Index] DataAnnotation in use on the model. If I register an account in the MVC Web App for the first time and don't specify a value for ITIN, SSN or FEIN, the first registration succeeds. If I register subsequent accounts, I get a run-time error advising can't insert a duplicate entry for NULL.

What I would like to do, if possible, is only allow unique values if something is inputted by the end user except for NULLS. Any suggestions would be appreciated.

    [Column("ITIN")]
    [DataType(DataType.Text)]
    [Display(Name = "ITIN")]
    [MinLength(0), MaxLength(11), StringLength(11, ErrorMessage = "The {0} must be no more than {2} characters in length and in the format of ###-##-####. This is optional.", MinimumLength = 0)] // XXX-XX-XXXX
    [Index("UQ_UserInfo_ITIN", IsClustered = true, IsUnique = true)]
    [DisplayFormat(ConvertEmptyStringToNull = true)]
    [Required(AllowEmptyStrings = true,
              ErrorMessage = "The {0} is required. Please enter in a valid {0}."),
              ConcurrencyCheck]
    public string ITIN { get; set; }