在LINQ中，投影是将对象转换为新形式的操作，该新形式仅包含随后将要使用的那些属性。通过使用投影，开发人员可以创建从每个对象构建的新类型。你被允许项目财产并在其上进行数学函数，你也可以投射原始对象，而不将其转化。
在LINQ中，可以使用以下投影操作：
- 选择
- 全选
SelectMany运算符
SelectMany运算符返回基于转换函数的值序列，然后将它们变成一个序列。换句话说，可以说，当您要从多个集合中选择值时，或者想要从列表列表中获得结果并希望显示为单个序列时，将使用SelectMany运算符。
查询语法中的SelectMany：在查询语法中，SelectMany运算符的工作是通过使用多个from子句来实现的。如下例所示。
例子：
// C# program to find the languages
// known by the employee
using System;
using System.Linq;
using System.Collections.Generic;
// Employee details
public class Employee {
public int emp_id
{
get;
set;
}
public string emp_name
{
get;
set;
}
public string emp_gender
{
get;
set;
}
public string emp_hire_date
{
get;
set;
}
public int emp_salary
{
get;
set;
}
public List emp_lang
{
get;
set;
}
}
class GFG {
// Main method
static public void Main()
{
List emp = new List() {
new Employee() {emp_id = 209, emp_name = "Anjita", emp_gender = "Female", emp_hire_date = "12/3/2017", emp_salary = 20000, emp_lang = new List{"C#", "VB"} },
new Employee() {emp_id = 210, emp_name = "Soniya", emp_gender = "Female", emp_hire_date = "22/4/2018", emp_salary = 30000, emp_lang = new List{ "Java"} },
new Employee() {emp_id = 211, emp_name = "Rohit", emp_gender = "Male", emp_hire_date = "3/5/2016", emp_salary = 40000, emp_lang = new List{ "C++", "SQL"} },
new Employee() {emp_id = 212, emp_name = "Supriya", emp_gender = "Female", emp_hire_date = "4/8/2017", emp_salary = 40000, emp_lang = new List{"Python", "C", "PHP"} },
new Employee() {emp_id = 213, emp_name = "Anil", emp_gender = "Male", emp_hire_date = "12/1/2016", emp_salary = 40000, emp_lang = new List{"HTML", "JQuery"} },
new Employee() {emp_id = 214, emp_name = "Anju", emp_gender = "Female", emp_hire_date = "17/6/2015", emp_salary = 50000, emp_lang = new List{"JavaScript", "Perl"} },
};
// Query to find the languages
// known by the employee
var res = from e in emp
from e2 in e.emp_lang
select e2;
Console.WriteLine("Languages known by all the employees are:");
foreach(var val in res)
{
Console.WriteLine(val);
}
}
}
输出：
Languages known by all the employees are:
C#
VB
Java
C++
SQL
Python
C
PHP
HTML
JQuery
JavaScript
Perl
方法语法中的SelectMany ： Queryable和Enumerable类中都存在SelectMany方法，并且C＃和VB.Net语言都支持SelectMany方法。
例子：
// C# program to find the languages
// known by the employee
using System;
using System.Linq;
using System.Collections.Generic;
// Employee details
public class Employee {
public int emp_id
{
get;
set;
}
public string emp_name
{
get;
set;
}
public string emp_gender
{
get;
set;
}
public string emp_hire_date
{
get;
set;
}
public int emp_salary
{
get;
set;
}
public List emp_lang
{
get;
set;
}
}
public class GFG {
// Main method
static public void Main()
{
List emp = new List() {
new Employee() {emp_id = 209, emp_name = "Anjita", emp_gender = "Female", emp_hire_date = "12/3/2017", emp_salary = 20000, emp_lang = new List{"C#", "VB"} },
new Employee() {emp_id = 210, emp_name = "Soniya", emp_gender = "Female", emp_hire_date = "22/4/2018", emp_salary = 30000, emp_lang = new List{ "Java"} },
new Employee() {emp_id = 211, emp_name = "Rohit", emp_gender = "Male", emp_hire_date = "3/5/2016", emp_salary = 40000, emp_lang = new List{ "C++", "SQL"} },
new Employee() {emp_id = 212, emp_name = "Supriya", emp_gender = "Female", emp_hire_date = "4/8/2017", emp_salary = 40000, emp_lang = new List{"Python", "C", "PHP"} },
new Employee() {emp_id = 213, emp_name = "Anil", emp_gender = "Male", emp_hire_date = "12/1/2016", emp_salary = 40000, emp_lang = new List{"HTML", "JQuery"} },
new Employee() {emp_id = 214, emp_name = "Anju", emp_gender = "Female", emp_hire_date = "17/6/2015", emp_salary = 50000, emp_lang = new List{"JavaScript", "Perl"} },
};
// Finding the languages known by the employee
// Using SelectMany method
var res = emp.SelectMany(a => a.emp_lang);
Console.WriteLine("Languages known by all the employees are:");
foreach(var val in res)
{
Console.WriteLine(val);
}
}
}
输出：
Languages known by all the employees are:
C#
VB
Java
C++
SQL
Python
C
PHP
HTML
JQuery
JavaScript
Perl