C# 选择中的 LINQ 选择
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LINQ Select within a Select
提问by DK ALT
I am trying to query a collection that contains Employeeinformation. When I query that collection I would like to return an enumeration of objects where each object has two fields:
我正在尝试查询包含员工信息的集合。当我查询该集合时，我想返回一个对象枚举，其中每个对象都有两个字段：
- Name
- ManagerName
- 姓名
- 经理姓名
(Note that every Manager is also an Employee!)
（请注意，每个经理也是员工！）
Now, here's the problem I am having. When I do a selectwithin a select, the value of the ManagerNamefield that is returned on each object is:
现在，这就是我遇到的问题。当我在 aselect中执行 a 时select，ManagerName每个对象上返回的字段的值是：
System.Data.Common.Internal.Materialization.CompensatingCollection<string>
Here's the query:
这是查询：
var query =
from e in db.Employees
select new
{
Name = e.Name,
ManagerName =
from em2 in db.Employees
where (em2.EmployeeID == e.ManagerID)
select em2.Name
};
Specifically, when I look at the value of ManagerName, I see that it is an enumeration that yields a single item. And that the single item is a string that contains the name of the Manager. So, I think I'm close.
具体来说，当我查看 的值时ManagerName，我看到它是一个生成单个项目的枚举。并且单个项目是一个包含经理姓名的字符串。所以，我想我很接近了。
Question:How can I change my query so that instead it returns an enumeration of objects where each object simply has two string fields, Nameand ManagerName?
问题：如何更改我的查询，以便它返回对象的枚举，其中每个对象只有两个字符串字段，Name而ManagerName?
采纳答案by Daniel Hilgarth
Try this:
尝试这个：
var query = from e in db.Employees
select new
{
Name = e.Name,
ManagerName = db.Employees
.Where(x => x.EmployeeID == e.ManagerID)
.Select(x => x.Name).SingleOrDefault()
};
However, if you correctly mapped your database with EF(which I suppose you are using), you should have a navigation property you can utilize:
但是，如果您使用EF（我想您正在使用）正确映射了您的数据库，您应该有一个可以使用的导航属性：
var query = from e in db.Employees
select new
{
Name = e.Name,
ManagerName = e.Manager.Name
};
回答by D Stanley
Looks like a self-join should work:
看起来自连接应该可以工作：
var query = from e in db.Employees
join m in db.Employees on e.ManagerID equals m.EmployeeID
select new
{
Name = e.Name,
ManagerName = m.Name
};
