LINQ:从每个组中选择“最新”

Question

所以，我知道我很生疏，但我从未想过尽管花了几个小时尝试不同的解决方案，但这会如此困难。 在消除特定子句然后将其绑定到网格后，我正在尝试从每个组中选择最新记录。 基本上没有运气...这是我离开的地方：

var widgets = db.Updates
            .GroupBy(c => c.widgetType)
            .SelectMany(s => s)
            .Where(c => c.Sold.Equals(false))
            .OrderByDescending(x => x.TimeStamp)
            .FirstOrDefault();
        var list = new List<Update>() {widgets};
            widgetsGrid.DataSource = list;            
            widgetsGrid.DataBind();

由于我收到数据类型错误，我将强制转换为列表，目前它仅返回所有记录的最后一项，而不是每个组中的最后一项。

提前感谢您的任何帮助!!

Answer 1

OrderByDescending应该在每个组的级别上。您也不需要SelectMany因为这会将组重新加入到平面列表中。

var widgets = Updates
        .GroupBy(c => c.widgetType)
        .Where(c => c.Sold.Equals(false))
        .Select(x => x.OrderByDescending(y => y.TimeStamp).First());

Answer 2

         var widgets = db.Updates
        .GroupBy(c => c.widgetType)
        .SelectMany(s => s)
        .Where(c => c.Sold.Equals(false))
        .OrderByDescending(x => x.TimeStamp)
        .Select(y=>y.First());
    var list = new List<Update>() {widgets};
        widgetsGrid.DataSource = list;            
        widgetsGrid.DataBind();

Answer 3

似乎您需要做的关键事情是按顺序并从列表中选择第一项。下面是一个示例程序：

class Program
{
    static void Main(string[] args)
    {
        var foodOrders = new List<FoodOrder>
        {
            new FoodOrder { FoodName = "hotdog", OrderDate = new DateTime(2016, 7, 7) },
            new FoodOrder { FoodName = "hamburger", OrderDate = new DateTime(2016, 7, 6) },
            new FoodOrder { FoodName = "taco", OrderDate = new DateTime(2016, 7, 5) },
        };
        var mostRecentFoodOrder = foodOrders.OrderByDescending(f => f.OrderDate).First().FoodName;
        Console.WriteLine(mostRecentFoodOrder);
        //cmd
        //hotdog
        //Press any key to continue . . .
    }
}
class FoodOrder
{
    public string FoodName { get; set; }
    public DateTime OrderDate { get; set; }
}

Answer 4

var widgets = db.Updates
          .Where(c => c.Sold.Equals(false))
          .GroupBy(c => c.widgetType)       
      .Select(x => x.OrderByDescending(y => y.TimeStamp).First()).ToList();
       widgetGrid.DataSource = widgets;
       widgetGrid.DataBind();

这个问题我在网上

看到过一百次......这对我有用。 由易卜拉欣@Ahmad提供并非常感谢