使用 linq 查询嵌套列表

Posted 2023-04-12

【中文标题】使用 linq 查询嵌套列表

【英文标题】：Querying a nested list using linq

【发布时间】：2017-11-29 00:03:11

【问题描述】：

我是初学者学习 linq。如何使用 linq 查询列表对象

var dbUserSettings = new List<UserSetting>   
    new UserSetting  
        UserId = "abcxyz@123", 
        Applications = new List<Application>  
            new Application  
                ApplicationID = "application123", 
                ApplicationName = "ProtocolArchiving", 
                Settings = new List<Setting>  
                    new Setting  
                        SettingId = "setting123", 
                        SettingKey = "RefreshInterval", 
                        SettingValue = "20", 
                        IsActive = "true", 
                        UpdatedOn = "2017-06-22", 
                        SettingLabel = "PageRefresh"     ,

    new UserSetting  
        UserId = "abcxyz@345", 
        Applications = new List<Application>  
            new Application  
                ApplicationID = "application345", 
                ApplicationName = "ProtocolArchiving", 
                Settings = new List<Setting>  
                    new Setting  
                    SettingId = "setting456", 
                        SettingKey = "UploadSetting", 
                        SettingValue = "20", 
                        IsActive = "true", 
                        UpdatedOn = "2017-06-22", 
                        SettingLabel = "Upload"     ,
    new UserSetting  
        UserId = "abcxyz@567", 
        Applications = new List<Application>  
            new Application  
                ApplicationID = "application678", 
                ApplicationName = "ProtocolArchiving", 
                Settings = new List<Setting>  
                    new Setting  
                        SettingId = "setting789", 
                        SettingKey = "DownloadSetting", 
                        SettingValue = "20", 
                        IsActive = "true", 
                        UpdatedOn = "2017-06-22", 
                        SettingLabel = "Download"     
     ;

 var response = dbUserSettings.Where(e => e.UserId == userID)
                   .Select(dbsetting => new UserSettingViewModel
                   
                       SettingKey = dbsetting.Applications.Single<Setting>(s=> s == )

                   )
                   .ToArray();

我正在查询与我的用户 ID 匹配的设置键。

编辑：

错过了几件事要提。我在这里尝试了几件事

SettingKey = dbsetting.Applications.FirstOrDefault().Select(sk => sk.Settings.FirstOrDefault()?.SettingKey);

报错如下

我的应用程序类如下所示。

public class Application

    public string ApplicationID  get; set; 
    public string ApplicationName  get; set; 
    public List<Setting> Settings  get; set; 

【问题讨论】：

I am querying for settingkey which matches with my userID. 对你有好处，实际上是什么问题？

你的问题是什么？有什么意想不到的结果吗？错误？请更具体地说明您要做什么。

SettingKey = dbsetting.Applications.FirstOrDefault()?.Select(sk => sk.Settings.FirstOrDefault()?.SettingKey)

我认为 select many 是您正在寻找的东西。 ***.com/questions/958949/…

@LaurentLequenne 这甚至不会编译，因为FirstOrDefault 返回一个没有Select 的Application 实例。

【参考方案1】：

因为您只对每个 UserSetting 的唯一一个 Application 感兴趣，我想您需要这个：

var response = dbUserSettings.Where(e => e.UserId == userID)
    .Select(dbsetting => new UserSettingViewModel
    
         SettingKey = dbsetting.Applications.FirstOrDefault()?.Settings.FirstOrDefault()?.SettingKey
    ;

这将只返回每个UserSetting 内的第一个（并且可能也是唯一的）Application 中的第一个（可能是唯一的）Setting。如果两者之间的任何列表为空（Applications 或 Settings），则 SettingKey 将为 null。