Question

我正在尝试将以下SQL查询转换为LINQ，但我得到一个“对象引用未设置为对象的实例。”

SQL QUERY

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 OUTPUT

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 QUERY

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();

我想问题就在这里，因为我正在尝试加入两个NULL值，但我无法弄清楚如何解决这个问题......

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进行空值检查，例如：

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

Answer 2

您还可以尝试实例化新对象，而不是使用default null。语法类似于partial1.DefaultIfEmpty(new AllObjective())

LINQ - 具有可空值的多个左连接

2 个答案: