C# Linq 左外聯

假設我們有一個Student類，具有姓名、年齡、班級Id三個屬性
還有一個Class類，具有Id，班級名兩個屬性

現要查找學生姓名、年齡和其所在班級名，因此需要做一個Student表左聯Class表的操作

方式一：

var results = from s in studentsjoin c in classes on s.ClassId equals c.Id into scsfrom sc in scs.DefaultIfEmpty()select new { Name = s.Name, Age = s.Age, ClassName = sc?.ClassName ?? string.Empty };

方式二：

var results = students.GroupJoin(classes,s => s.ClassId,c => c.Id,(s, cs) => new{Name = s.Name,Age = s.Age,ClassName = cs.ElementAtOrDefault(0) is null ? string.Empty : cs.ElementAt(0).ClassName});

完整代碼演示：

using System; using System.Collections.Generic; using System.Linq;public class Student {public string Name { get; set; }public int Age { get; set; }public int ClassId { get; set; } }public class Class {public int Id { get; set; }public string ClassName { get; set; } }public class Program {public static void Main(){List<Student> students = new List<Student>(){new Student(){Name = "B", Age = 18, ClassId = 1},new Student(){Name = "C", Age = 19, ClassId = 2},new Student(){Name = "A", Age = 20, ClassId = 3},new Student(){Name = "D", Age = 20, ClassId = 4},new Student(){Name = "E", Age = 20, ClassId = 5},};List<Class> classes = new List<Class>(){new Class(){ Id = 1, ClassName = "1班"},new Class(){ Id = 2, ClassName = "2班"},new Class(){ Id = 3, ClassName = "3班"},};//方式一：//var results = from s in students// join c in classes on s.ClassId equals c.Id into scs// from sc in scs.DefaultIfEmpty()// select new { Name = s.Name, Age = s.Age, ClassName = sc?.ClassName ?? string.Empty };//方式二：var results = students.GroupJoin(classes,s => s.ClassId,c => c.Id,(s, cs) => new{Name = s.Name,Age = s.Age,ClassName = cs.ElementAtOrDefault(0) is null ? string.Empty : cs.ElementAt(0).ClassName});foreach (var result in results){Console.WriteLine($"{result.Name} {result.Age} {result.ClassName}");}} }

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