LINQ离开了连接数据集和分组依据

Question

我的数据看起来像这样

1. EOC - 案件ID，入院，出院
2. PrinDiag - 每个EOC一个

3. OtherDiag - 每个EOC 0,1或多个

    var results = from dtE in ds.Tables["EOC"].AsEnumerable()
          from dtPD in ds.Tables["PrinDiag"].AsEnumerable()
          .Where(t => dtE.Field<string>("case_id") ==  t.Field<string>("case_id") )
          .DefaultIfEmpty()
          .GroupBy(t => t.Field<string>("case_id"))
          from dtOD in ds.Tables["OtherDiag"].AsEnumerable()
          .Where(t => dtE.Field<string>("case_id") == t.Field<string>("case_id"))                                                 
          .DefaultIfEmpty()
          .GroupBy(t => t.Field<string>("case_id")).DefaultIfEmpty()
        select new { dtE, dtPD, dtOD };
   

我需要从这些数据创建XML。

<eocdate>
<EOC>test</EOC>
<admission>22/06/2013</admission>
<patient>
  <facility_patient_id>12345</facility_patient_id>
  <first_name>Matthu</first_name>
  <last_name>MOmmy</last_name>
</patient>
<assessment>
  <newborn_weight />
</assessment>
<principal_diagnosis>
  <principal_diagnosis_code>123   </principal_diagnosis_code>
  <principal_diagnosis_status>0</principal_diagnosis_status>
</principal_diagnosis>
<other_diagnosis>
  <other_diagnosis_code>435   </other_diagnosis_code>
  <other_diagnosis_status>0</other_diagnosis_status>
</other_diagnosis>
<other_diagnosis>
  <other_diagnosis_code>345   </other_diagnosis_code>
  <other_diagnosis_status>0</other_diagnosis_status>
</other_diagnosis>
<other_diagnosis>
  <other_diagnosis_code>1231  </other_diagnosis_code>
  <other_diagnosis_status>0</other_diagnosis_status>
</other_diagnosis>
</eocdate>

我收到“值不能为空。\ r \ nParameter name：row”

请帮忙。

谢谢，