如何使用 C# 中的 Lambda 表達式僅返回沒有來自另一個表的行的行 (How to return only rows where no rows from another table using Lambda Expressions in C#)

問題描述

如何使用 C# 中的 Lambda 表達式僅返回沒有來自另一個表的行的行 (How to return only rows where no rows from another table using Lambda Expressions in C#)

我有兩張桌子；Notification 和 SeenBy。

Notification：

Notification.Id
Notification.Timestamp
Notification.Message

SeenBy

SeenBy.Id
SeenBy.NotificationId
SeenBy.ClientId
SeenBy.Timestamp

我需要獲取行使用輔助表 SeenBy 來自以前未見過的表 Notification (Count=0)。

Linq 查詢或 Lambda 表達式是什麼？

參考解法

方法 1:

You can try something like this:

var seenNotificationIds = seenByCollection.Select(x => x.NotificationId).Distinct();
var seenNotifications = notifications.Where(x => seenNotificationIds.Contains(x.Id));
var notSeenNotifications = notifications.Except(seenNotifications);
notSeenNotifications.Dump();

方法 2:

You can use this code:

var allNotificationObjects = ...; // all your notification objects
var seenByObjects = ...; // all seen by objects

var seenNotificationIds = seenByObjects
                         .Select(item => item.NotificationId)
                         .Distinct()
                         .ToHashSet();   

var result = allNotificationObjects
            .Where(item => !seenNotificationsIds.Contains(item.Id))
            .ToList(); // all notifications which are not seen

(by Nime Cloud、Mahdi、Roman)

參考文件

1. How to return only rows where no rows from another table using Lambda Expressions in C# (CC BY‑SA 2.5/3.0/4.0)