C# List left join

public class Test1
{
public int ID { get; set; }
public string Name { get; set; }
}

public class Test2
{
public int ID { get; set; }
public int Age { get; set; }
}

//添加集合

List<Test1> test1 = new List<Test1>();
List<Test2> test2 = new List<Test2>();

test1.Add(new Test1() { ID = 1, Name = "zbb" });
test1.Add(new Test1() { ID = 2, Name = "czf" });

test2.Add(new Test2() { ID = 1, Age = 33 });

//左连接

var result = from l in test1
join r in test2 on l.ID equals r.ID into temp
from lr in temp.DefaultIfEmpty()
select new { l.ID, l.Name, Age = temp.Select(T => T.Age).FirstOrDefault() };

// 返回结果

ID Name Age

1   zbb    33

2   czf     0