c# – Linq在可空字段上离开了连接

发布时间：2020-12-15 23:36:44 所属栏目：百科来源：网络整理

导读：在下面的示例中,我有entity1,其中user_id是可以为null的int,有时它确实为null. Entity2类似于通过user_id将entity1连接到entity3的桥梁,以获取person_id并最终获得相应的fullname. 我的目标是编写一个linq查询,其中如果entity1的user_id为null,则不抛出NullR

在下面的示例中,我有entity1,其中user_id是可以为null的int,有时它确实为null. Entity2类似于通过user_id将entity1连接到entity3的桥梁,以获取person_id并最终获得相应的fullname.

我的目标是编写一个linq查询,其中如果entity1的user_id为null,则不抛出NullReferenceException,但为fullname属性获取null.

鉴于以下实体,我希望我的查询返回：

> codeOfEntity =“21006.040”,fullname = null
> codeOfEntity =“14006.010”,fullname =“John Smith”
> codeOfEntity =“21006.020”,fullname = null
> codeOfEntity =“35716.001”,fullname =“Dave Doe”
> codeOfEntity =“11153.013”,fullname =“Zach White”

实体：

class entity1
{
    public int? user_id { get; set; }
    public string codeOfEntity { get; set; }
}

class entity2
{
    public int user_id { get; set; }
    public int person_id { get; set; }
}

class entity3
{
    public int person_id { get; set; }
    public string fullname { get; set; }
}

并填写如下：

var listOfEntity1 = new List<entity1>()
{
    new entity1() { user_id = null,codeOfEntity = "21006.040" },new entity1() { user_id = 10,codeOfEntity = "14006.010" },new entity1() { user_id = null,codeOfEntity = "21006.020" },new entity1() { user_id = 1,codeOfEntity = "35716.001"},new entity1() { user_id = 4,codeOfEntity = "11153.013" }
};

var listOfEntity2 = new List<entity2>()
{
    new entity2() { user_id = 1,person_id = 100 },new entity2() { user_id = 4,person_id = 400 },new entity2() { user_id = 10,person_id = 1000 }
};

var listOfEntity3 = new List<entity3>()
{
    new entity3() { person_id = 100,fullname = "John Smith" },new entity3() { person_id = 400,fullname = "Dave Doe" },new entity3() { person_id = 1000,fullname = "Zach White" }
};

我的查询抛出NullReferenceException：

var result = (from e1 in listOfEntity1
              join e2 in listOfEntity2 on e1.user_id equals e2.user_id into e2Group
              from e2 in e2Group.DefaultIfEmpty()
              join e3 in listOfEntity3 on e2.person_id equals e3.person_id into e3Group
              from e3 in e3Group.DefaultIfEmpty()
              select new { e1.codeOfEntity,e3.fullname }).ToList();

谢谢.

解决方法

使用c#6.0的空传播

var result = (from e1 in listOfEntity1
              join e2 in listOfEntity2 on e1.user_id equals e2.user_id into e2Group
              from e2 in e2Group.DefaultIfEmpty()
              join e3 in listOfEntity3 on e2?.person_id equals e3.person_id into e3Group
              from e3 in e3Group.DefaultIfEmpty()
              select new { e1.codeOfEntity,e3?.fullname }).ToList();

注意连接条件和选择的简单更改.在连接中将e2.person_id更改为e2？.person_id,在select中将e3.fullname更改为e3？.fullname.

但是,如果你不能使用c#6.0,这里有另一种选择

var result = (from e1 in listOfEntity1
              join e2 in listOfEntity2 on e1.user_id equals e2.user_id into e2Group
              from e2 in e2Group.DefaultIfEmpty()
              join e3 in listOfEntity3 on (e2==null?-1:e2.person_id) equals e3.person_id into e3Group
              from e3 in e3Group.DefaultIfEmpty()
              select new { e1.codeOfEntity,fullname=(e3==null?null:e3.fullname) }).ToList();

（编辑：李大同）

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