未将对象引用设置到对象的实例。该怎么处理

2013-01-25

未将对象引用设置到对象的实例。var ps from d in dt.AsEnumerable()join r in re on d.Fieldstring(p

未将对象引用设置到对象的实例。

  var ps = from d in dt.AsEnumerable()
                             join r in re on d.Field<string>("part_no") equals r.IFSID into joinedEmpDept
                             from dept in joinedEmpDept.DefaultIfEmpty() 
                             select new
                                        {
                                            IFSID = d.Field<string>("part_no"),/*从这边开始就报错了*/
                                            Description = d.Field<string>("Description"),
                                            Num = d.Field<decimal>("count_variance"),
                                            Unit = d.Field<string>("unit_meas"),
                                            IN_TRANSIT = d.Field<decimal>("IN_TRANSIT"),
                                            Price = d.Field<decimal>("price"),
                                            MAX = dept.MAX,
                                            MIN = dept.MIN
                                        };


//这样写就不报错
//但是这样写貌似不是左连接，
//dt.AsEnumerable()里面原来有十条数据，用下面的方法只能查询出5条
//既是两个表中都有的5条，另外5条没了
                      var ps = from d in dt.AsEnumerable() 
 
                             join r in re on d.Field<string>("part_no") equals r.IFSID 
                             select new
                                        {
                                            IFSID = d.Field<string>("part_no"),
                                            Description = d.Field<string>("Description"),
                                            Num = d.Field<decimal>("count_variance"),
                                            Unit = d.Field<string>("unit_meas"),
                                            IN_TRANSIT = d.Field<decimal>("IN_TRANSIT"),
                                            Price = d.Field<decimal>("price"),
                                            MAX = r.MAX,
                                            MIN = r.MIN
                                        };

求linq中左连接语法，要能把左边的表中数据全部查询出来的
[最优解释]
如果是左连，MAX = dept.MAX dept有可能为空吗？。。。
[其他解释]

引用:

如果是左连，MAX = dept.MAX dept有可能为空吗？。。。

恩，后来添加了

 var ps = from d in dt.AsEnumerable()
                             join r in re on d.Field<string>("part_no") equals r.IFSID into joinedEmpDept 
 
                             from dept in joinedEmpDept.DefaultIfEmpty()
                             select new
                                        {
                                            IFSID = d.Field<string>("part_no"),
                                            Description = d.Field<string>("Description"),
                                            Num = d.Field<decimal>("count_variance"),
                                            Unit = d.Field<string>("unit_meas"),
                                            IN_TRANSIT = d.Field<decimal>("IN_TRANSIT"),
                                            Price = d.Field<decimal>("price"),
                                            MAX = dept.MAX != null ? dept.MAX : 0,
                                            MIN = dept.MIN != null ? dept.MIN : 0
                                        };

我调试的时候发现整个joinedEmpDept就为null,不知道为什么
[其他解释]
最后两行写成这样就可以了

MAX = dept != null ? dept.MAX : 0,
MIN = dept != null ? dept.MIN : 0,

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未将对象引用设置到对象的实例。该怎么处理