Linq 获取组中的第一项和最后一项（如果存在）

我有一个场景，其中我的数据如下所示

Books --------------------------------- title | returnedDate Great Gatsby | 2015-05-04 Great Gatsby | 2015-03-22 Great Gatsby | 2015-01-11 Life of PI | 2015-04-04 Life of PI | 2015-04-02 Clean Code | 2015-06-05

var books = dbContext.Books
    .GroupBy(b => b.title)
    .Select(g => g.OrderDescending().FirstOrDefault());

如果最后一项也存在，我如何获得它

我的最终结果如下：

Books --------------------------------- title | returnedDate Great Gatsby | 2015-05-04 Great Gatsby | 2015-01-11 Life of PI | 2015-04-04 Life of PI | 2015-04-02 Clean Code | 2015-06-05 书 --------------------------------- 标题|返回日期 《了不起的盖茨比》2015-05-04 《了不起的盖茨比》2015-01-11 PI的寿命| 2015-04-04 PI的寿命| 2015-04-02 清洁规范| 2015-06-05 如果你真的像你要求的那样想要它，那么它就变得有点困难：

var books = dbContext.Books
    .GroupBy(b => b.title)
    .Select(g=>new {
       title=g.Key,
       returnedDate=g.OrderByDescending(x=>x).FirstOrDefault()
     }).Concat(
      dbContext.Books
      .GroupBy(b => b.title)
      .Where(g=>g.Count()>1)
      .Select(g=>new {
         title=g.Key,
         returnedDate=g.OrderBy(x=>x).FirstOrDefault()
       })
      ).OrderBy(c=>c.title).ThenDescendingBy(c=>c.returnedDate);

---

恶心。也许是一个更好的方法，但首先我想到了这一点。

经过一点努力，我想出了这个。不确定在处理大型数据表时这会有多高效

        [Test]
    public void FirstLastLinq()
    {
        var books = new List<Book> 
        { 
            new Book { Title = "Great Gatsby", Returned=new DateTime(2015,04,03) },
            new Book { Title = "Great Gatsby", Returned=new DateTime(2015,04,02) },
            new Book { Title = "Great Gatsby", Returned=new DateTime(2015,04,01) },
            new Book { Title = "Life of PI", Returned=new DateTime(2015,03,05) },
            new Book { Title = "Life of PI", Returned=new DateTime(2015,03,04) },
            new Book { Title = "Clean Code", Returned=new DateTime(2015,02,02) },
        };

        var newBooks = books.GroupBy(b => b.Title).SelectMany(g => g.OrderByDescending(b => b.Returned)
            .Where(b1 => b1.Returned == g.Min(b2 => b2.Returned) || 
                (b1.Returned == g.Max(b3 => b3.Returned) && g.Min(b4 => b4.Returned) != g.Max(b5 => b5.Returned))));

        Assert.IsNotNull(newBooks);

    }
    private class Book 
    {
        public string Title { get; set; }
        public DateTime Returned { get; set; }
    }

[测试]
public void FirstLastLinq（）
{
var books=新列表
{ 
新书{Title=“Great Gatsby”，Returned=new DateTime（2015,04,03）}，
新书{Title=“Great Gatsby”，Returned=new DateTime（2015,04,02）}，
新书{Title=“Great Gatsby”，Returned=new DateTime（2015,04,01）}，
新书{Title=“PI的生命”，返回=新日期时间（2015,03,05）}，
新书{Title=“PI的生命”，返回=新日期时间（2015,03,04）}，
新书{Title=“Clean Code”，返回=新日期时间（2015,02,02）}，
};
var newBooks=books.GroupBy（b=>b.Title）。SelectMany（g=>g.OrderByDescending（b=>b.Returned）
其中（b1=>b1.Returned==g.Min（b2=>b2.Returned）|
（b1.Returned==g.Max（b3=>b3.Returned）和&g.Min（b4=>b4.Returned）！=g.Max（b5=>b5.Returned））；
Assert.IsNotNull（newBooks）；
}
私塾
{
公共字符串标题{get；set；}
返回的公共日期时间{get；set；}
}

获取第一个和最后一个归还日期，然后归还归还日期等于以下日期的书籍：

from b in dbContext.Books
group b by b.title into bg
let first = bg.OrderByDescending (b => b.returnedDate).FirstOrDefault().returnedDate
let last = bg.OrderBy (b => b.returnedDate).FirstOrDefault().returnedDate
from b in bg
where b.returnedDate == first || b.returnedDate == last
orderby b.title, b.returnedDate
select b

---

哎哟，看起来很吓人。无论如何，谢谢你的回答。谢谢你的建议格特。我想返回原始的Book对象，而不是匿名对象

from b in dbContext.Books
group b by b.title into bg
let first = bg.OrderByDescending (b => b.returnedDate).FirstOrDefault().returnedDate
let last = bg.OrderBy (b => b.returnedDate).FirstOrDefault().returnedDate
from b in bg
where b.returnedDate == first || b.returnedDate == last
orderby b.title, b.returnedDate
select b