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 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
                                 });

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}");
        }
    }
}