使用linq查询嵌套列表

Question

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userID匹配的settingkey。

编辑：

错过了几件事要提。我在这里尝试了一些事情

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

错误如下

我的应用程序类看起来像这样。

public class Application
{
    public string ApplicationID { get; set; }
    public string ApplicationName { get; set; }
    public List<Setting> Settings { get; set; }
}

Answer 1

由于您只对每Application UserSetting感兴趣，我认为您需要这个：

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Setting内的第一个（也可能是唯一的）Application中的第一个（也可能只是）UserSetting。如果其间的任何列表为空（Applications或Settings）SettingKey将为null。