使用 linq 查询嵌套列表答案

【问题标题】：Querying a nested list using linq使用 linq 查询嵌套列表
【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。 stackoverflow.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。

标签： c# linq linq-to-objects

【解决方案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。

【讨论】：