Lets say I have a StartDate and an EndDate and I wnt to check if the EndDate is not more then 3 months apart from the Start Date

public class DateCompare : ValidationAttribute { public String StartDate { get; set; } public String EndDate { get; set; }

    //Constructor to take in the property names that are supposed to be checked
    public DateCompare(String startDate, String endDate)
    {
        StartDate = startDate;
        EndDate = endDate;
    }

    public override bool IsValid(object value)
    {
        var str = value.ToString();
        if (string.IsNullOrEmpty(str))
            return true;

        DateTime theEndDate = DateTime.ParseExact(EndDate, "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture);
        DateTime theStartDate = DateTime.ParseExact(StartDate, "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture).AddMonths(3);
        return (DateTime.Compare(theStartDate, theEndDate) > 0);
    }
}

and I would like to implement this into my validation

[DateCompare("StartDate", "EndDate", ErrorMessage = "The Deal can only be 3 months long!")]

I know I get an error here... but how can I do this sort of business rule validation in asp.net mvc