按多个列分组并选择特定列

Question

如何在 LINQ 中进行 GroupBy 多列操作，并且我想获取一些列：

public class Notification
{
    [Key]
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
    public long NotificationId { set; get; }


    public NotificationType NotificationType { set; get; }


    public int UserId { get; set; }
    public UserProfile UserProfile { get; set; }


    public DateTime CreateDate { set; get; }



    public bool Notice { set; get; }


    public long NewsId { set; get; }


    public int RecipientId { get; set; }
}

我写了以下代码，但是显示所有记录：

var x=(from n in _Notification
                where n.RecipientId == id
                orderby n.CreateDate descending, n.UserId
                group new { n.UserId, n.NewsId}
                by new { n.UserId, n.NewsId, n.Notice, n.NotificationType, n.CreateDate, n.NotificationId, n.UserProfile } into g
                select new NotificationViewModel
                {
                    NewsId = g.Key.NewsId,
                    CauserId = g.Key.UserId,
                    CauserName = g.Key.UserProfile.Name,
                    CreateDate = g.Key.CreateDate,
                    Notice = g.Key.Notice,
                    NotificationId = g.Key.NotificationId,
                    NotificationType = g.Key.NotificationType,
                }).ToList();

我想根据UserId和NewsId进行分组

Answer 1

您会得到一个列表而不是分组列表，因为最后您有 select 语句，而最后应该有 group by。像这样更改您的代码：

var x=(from n in _Notification
            where n.RecipientId == id
            orderby n.CreateDate descending, n.UserId
            group new NotificationViewModel
            {
                NewsId = g.Key.NewsId,
                CauserId = g.Key.UserId,
                CauserName = g.Key.UserProfile.Name,
                CreateDate = g.Key.CreateDate,
                Notice = g.Key.Notice,
                NotificationId = g.Key.NotificationId,
                NotificationType = g.Key.NotificationType,
            }
            by new { n.UserId, n.NewsId}).ToList();