C# Linq 左外联

假设我们有一个Student类，具有姓名、年龄、班级Id三个属性
还有一个Class类，具有Id，班级名两个属性

现要查找学生姓名、年龄和其所在班级名，因此需要做一个Student表左联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});

完整代码演示：

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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