如何处理 linq 左连接中的空表

Question

我正在编写如下的 linq 查询：-

from eventDetails in CurrentDataContext.EventCarriers
join eventlog in CurrentDataContext.EventLogCarriers on eventDetails.EventID equals eventlog.EventId into hhh
from eventlog in hhh.DefaultIfEmpty()
join quiz in CurrentDataContext.QuizCarriers on eventDetails.EventID equals competition.EventId into aaa
from quiz in aaa.DefaultIfEmpty()

where eventDetails.EventID == eventId && eventDetails.EventStatusId == 1 && eventlog.CreatedBy == userId                                      
group new { eventDetails, quiz, eventlog } by new
{
  eventDetails.EventID,
  eventDetails.Name                                        
} into g
select new VenueModel
{
  Name = g.Key.Name,
  EventId = g.Key.EventID,
  StartDate = g.Key.StartDate,
  EndDate = g.Key.EndDate,
  EventDescr = g.Key.EventDescr,
  EventIconPath = g.Key.EventIconPath,
  EventImagePath = g.Key.EventImagePath,
  Prizes = (from a in g.Select(a => a.competitions)
                  group a by new
                  {
                      a.PrizeId,
                      a.PrizeIconPath,
                      a.CompetitionDescr,
                      a.PrizeImagePath,
                      a.EndDate,
                      a.StartDate,
                      a.Name,
                      a.EventId,
                      a.StatusId
                  } into x

                  select new CCompetitionModel
                  {
                      PrizeDescrption = x.Key.PrizeDescr,
                      PrizeIconPath = x.Key.PrizeIconPath,
                      PrizeId = x.Key.PrizeId,                                                              
                  }).ToList(),
}).FirstOrDefault();

但我收到类似 转换为值类型“System.Int32”的错误，因为具体化值为 null。结果类型的泛型参数或查询必须使用可为空的类型。

Answer 1

//Declare your class property nullable.
//For example, if you are getting start date null from database, then your property will be like following.


    public class Event
    {
      public DateTime? StartDate{get; set;}
      //add other properties of this class here..
    }