using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public void Main()

    {             

        var students = new List<Student>();

var courses =new List<Course>();

        students.Add(new Student{Id=3,Name="test"});

        students.Add(new Student{Id=4,Name="test2"});

        courses.Add(new Course{UserId=3,datetime=DateTime.Now});

        courses.Add(new Course{UserId=3,datetime=DateTime.Now.AddDays(1)});

        var query =courses.GroupBy(c => c.UserId).Select(g => g.OrderByDescending(c => c.datetime).FirstOrDefault());

        var res = (from u in students

              join c in query on u.Id equals c.UserId into j1

              from j2 in j1.DefaultIfEmpty()     

              select new { u.Name,j2}).ToList();

        res.ForEach(r => {Console.WriteLine(r.Name); if (r.j2 != null){ Console.WriteLine(r.j2.datetime);}});

    }   

}

​

public class Student

{

    public int Id{get;set;}

    public string Name{get;set;}

    public string Std{get;set;}

}

public class Course

{

    public int UserId{get;set;}

    public int Id{get;set;}

    public DateTime datetime{get;set;}

}

​