using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<People> listA = new List<People>

        {

            new People { ID = 1, Name = "Andy" },

            new People { ID = 2, Name = "Sam" }

        };

        List<People> listB = new List<People>

        {

            new People { ID = 1, Name = "Andy", Age = 10 }

        };

        var result = (from a in listA

                      join b in listB on new { a.ID, a.Name } equals new { b.ID, b.Name } into ab

                      from b in ab.DefaultIfEmpty()

                      select new People

                      {

                        ID = a.ID,

                          Name = a.Name,

                          Age = b?.Age

                      }

            ).ToList();

        foreach (var people in result)

        {

            Console.WriteLine($"{people.ID} {people.Name} {people.Age}");

        }

    }

}

​

public class People

{

    public int ID { get; set; }

    public string Name { get; set; }

    public int? Age { get; set; }

}