LINQ中的ToLookup运算符是一种扩展方法，用于从源提取一组键/值对。在这里，结果集合中的每个元素都是一个通用Lookup对象。Lookup对象保存与该键匹配的键和子项。
LINQ ToLookup运算符的语法
下面是使用LINQ ToLookup运算符将集合作为键/值对返回的语法。
C#代码
var Emp = objEmployee.ToLookup(x => x.Department);
In the above syntax, we are converting the collection of "objEmployee" to key/value pair list using the ToLookup operator.
方法语法中的LINQ ToLookup()运算符示例
下面是在方法语法中使用LINQ ToLookup()将输入集合项转换为键/值对列表的示例。
C#代码
using System; using System.Collections; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks;namespace ConsoleApp1 { class Program { static void Main(string[] args) { //create an objEmployee of the class Employee and create a list of Employee List objEmployee = new List() { new Employee(){ Name="Akshay Tyagi", Department="IT", Country="India"}, new Employee(){ Name="Vaishali Tyagi", Department="Marketing", Country="Australia"}, new Employee(){ Name="Arpita Rai", Department="HR", Country="China"}, new Employee(){ Name="Shubham Ratogi", Department="Sales", Country="USA"}, new Employee(){ Name="Himanshu Tyagi", Department="Operations", Country="Canada"} }; //objEmployee.ToLookup() method is used to print the value of the data in the pair/collection of items. var Emp = objEmployee.ToLookup(x => x.Department); Console.WriteLine("Grouping Employees by Department"); Console.WriteLine("---------------------------------"); foreach (var KeyValurPair in Emp) { Console.WriteLine(KeyValurPair.Key); // Lookup employees by Department foreach (var item in Emp[KeyValurPair.Key]) { Console.WriteLine("\t" + item.Name + "\t" + item.Department + "\t" + item.Country); } } Console.ReadLine(); } } class Employee { public string Name { get; set; } public string Department { get; set; } public string Country { get; set; } } }
在上面的示例中，我们使用ToLookup方法按部门对员工进行分组。因为ToLookup生成键/值对，所以我们在foreach循环中使用它，内部循环根据作为输入传递的键提取值。
输出：
查询语法中的LINQ ToLookup()示例
using System; using System.Collections; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks;namespace ConsoleApp1 { class Program { static void Main(string[] args) { List objEmployee = new List() { new Employee(){ Name="Ashish ", Department="Marketing", Country="India"}, new Employee(){ Name="John", Department="IT", Country="Australia"}, new Employee(){ Name="Kim", Department="Sales", Country="China"}, new Employee(){ Name="Marcia", Department="HR", Country="USA"}, new Employee(){ Name="John", Department="Operations", Country="Canada"} }; var emp = (from employee in objEmployee select employee).ToLookup(x => x.Department); Console.WriteLine("Grouping Employees by Department"); Console.WriteLine("---------------------------------"); foreach (var KeyValurPair in emp) { Console.WriteLine(KeyValurPair.Key); // Lookup employees by Department foreach (var item in emp[KeyValurPair.Key]) { Console.WriteLine("\t" + item.Name + "\t" + item.Department + "\t" + item.Country); } } Console.ReadLine(); } } class Employee { public string Name { get; set; } public string Department { get; set; } public string Country { get; set; } } }
输出：
