why ?? in c# web service placeholder

why ?? in c# web service placeholder

Here are my classes
public class Offer : ResponseBase
{
public long OfferID { get; set; }
public string OfferName { get; set; }
public long SessionID { get; set; }
public long UserID { get; set; }
public int InternalValue { get; set; }
public int ExternalValue { get; set; }
public byte Status { get; set; }
public List<OfferDetail> OfferDetails;
}
public class OfferDetail
{
public long OfferID { get; set; }
public long InterestID { get; set; }
public string RangeValue { get; set; }
public string Score { get; set; }
public string Importance { get; set; }
public string Range { get; set; }
}
public class Offers : ResponseBase
{
public List<Offer> offers;
}
public class ResponseBase
{
#region Properties
protected int errorCode = 0;
public int ErrorCode
{
get { return errorCode; }
set { errorCode = value; }
}
protected string message = string.Empty;
public string ErrorMessage
{
get { return message; }
set { message = value; }
}