Share point 2010 List where clause operators

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C#3.5

SharePoint

ListView

List<FertilVacancyItem> lstData = null;
          try
          {

              using (ContextSwitch cSwitch = new ContextSwitch())
              {
                  m_dataContext = cSwitch.GetFertilVacancyDataContext(m_webUrl);

                //  lstData = m_dataContext.FertilVacancy.Where(x => (x.CloseDate >= DateTime.Today) || (DBNull.Value.Equals(x.CloseDate) )).Where(x => (x.OpenDate <= DateTime.Today)).OrderByDescending(x => x.Id).ToList();
                  //if(m_dataContext.FertilVacancy.Where(x=> (DBNull.e x.CloseDate)
                  lstData = m_dataContext.FertilVacancy.Where(x => (x.OpenDate <= DateTime.Today)).OrderByDescending(x => x.Id).ToList();
               //   lstData = m_dataContext.FertilVacancy.OrderByDescending(x => x.Id).ToList();



              }
          }
          catch (Exception ex)
          {
              Logger.Instance.Error("VacancyManager_getFertilVacancyData", ex);
          }
          return lstData;
}

I am new bee in share point.
I need to check the close date is null.
if it is null i don't want to included the close date in where clause.
else if it is not null then close date must include in where clause.