c# - LINQ - 具有可为空值的多个左联接

Question

我正在尝试将以下 SQL 查询转换为 LINQ，但我收到“未设置对象实例的对象引用”。

SQL 查询

SELECT a.ID_TARGET,a.TARGET_NAME, b.OBJECTIVE_NAME, c.PERSPECTIVE_NAME FROM IL_OPR_MTR_TGT AS a
LEFT JOIN IL_OPR_MTR_OBJ b ON a.TARGET_OBJECTIVE_ID = b.ID_OBJECTIVE
LEFT JOIN IL_OPR_MTR_PRSPCT c ON b.OBJECTIVE_PERSPECT_ID = c.PERSPECTIVE_ID;

SQL 输出

ID_TARGET |TARGET_NAME  |OBJECTIVE_NAME |PERSPECTIVE_NAME
----------|-------------|---------------|----------------
    7     |TGT_01       |   TST02       |PERSPECTIVE_01
    8     |TGT01        |   TST02       |PERSPECTIVE_01
    9     |TARGET_02    |   TST02       |PERSPECTIVE_01
    10    |TARGET003AA  |   (null)      |(null)

LINQ 查询

var data = (from a in allTargets
join b in allObjectives on a.TARGET_OBJECTIVE_ID equals b.ID_OBJECTIVE into partial1
from b in partial1.DefaultIfEmpty()
join c in allPerspectives on b.OBJECTIVE_PERSPECT_ID equals c.PERSPECTIVE_ID into partial2
from c in partial2.DefaultIfEmpty()
select new
{
    ID_TARGET = a.ID_TARGET,
    TARGET_NAME = a.TARGET_NAME,
    OBJECTIVE_NAME = b != null ? b.OBJECTIVE_NAME : "",
    PERSPECTIVE_NAME = c != null ? c.PERSPECTIVE_NAME : ""
}).ToList();

我想问题出在这里，因为我正在尝试连接两个 NULL 值，但我不知道如何解决这个问题...

join c in allPerspectives on b.OBJECTIVE_PERSPECT_ID equals c.PERSPECTIVE_ID into partial2
from c in partial2.DefaultIfEmpty()

Answer 1

您必须在查询中对 b 进行空检查，例如:

join c in allPerspectives on (b == null ? "no-value" : b.OBJECTIVE_PERSPECT_ID) equals c.PERSPECTIVE_ID