内选择LINQ选择 [英] LINQ Select within a Select

问题描述

我想查询包含的员工信息的集合。当我查询该集合，我想返回对象的枚举，其中每个对象有两个字段：

• 名称


• ManagerName

（注意每个经理也是一个员工！）

现在，这里是我遇到的问题。当我做了选择中的选择的值 ManagerName 字段，它的每个对象返回的：

<预类=郎无prettyprint-覆盖> System.Data.Common.Internal .Materialization.CompensatingCollection<串GT;


下面的查询：

<预类=郎-CS prettyprint-覆盖> VAR的查询=从E在db.Employees

选择新的
{
名称= e.Name，
在db.Employees
从EM2 ManagerName =
，其中（em2.EmployeeID == e.ManagerID）
选择em2.Name
};


具体而言，当我在看的价值 ManagerName ，我看到它是产生单个项目的枚举。而且该单个项目是包含管理器的名称的字符串。所以，我觉得我很接近

问：我怎样才能改变我的查询，以便它返回的对象，其中每个枚举对象只是有两个字符串字段，名称和 ManagerName ？

解决方案

试试这个：

  VAR的查询=从E在db.Employees 
选择新的
 {
名称= e.Name，
 ManagerName = db.Employees 
。凡（X => x.EmployeeID == e.ManagerID）
。选择（X => x.Name）.SingleOrDefault（）
}; 
  

然而，如果你使用正确的 EF （我假设你正在使用），你应该有你可以使用导航属性：

  VAR的查询=从E在db.Employees 
选择新的
 {
名称= e.Name，
 ManagerName = e.Manager.Name 
}; 
  

I am trying to query a collection that contains Employee information. When I query that collection I would like to return an enumeration of objects where each object has two fields:

• Name
• ManagerName

(Note that every Manager is also an Employee!)

Now, here's the problem I am having. When I do a select within a select, the value of the ManagerName field that is returned on each object is:

System.Data.Common.Internal.Materialization.CompensatingCollection<string>

Here's the query:

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.

Question: How can I change my query so that instead it returns an enumeration of objects where each object simply has two string fields, Name and ManagerName?

解决方案

Try this:

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:

var query = from e in db.Employees
            select new
            {
                Name = e.Name,
                ManagerName = e.Manager.Name
            };

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