Querying a nested list using linq

Question

I am a beginner learning linq. How to query a list object using linq

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();

I am querying for settingkey which matches with my userID.

Edit:

Missed couple of things to mention. I have tried few things here

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

The error is as below

My application class looks like this.

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

`I am querying for settingkey which matches with my userID.` ok good for you, what's actually the question? — EpicKip, Jun 26 '17 at 07:26
And what´s your problem? Any unexpected results? Errors? Please be more specific on what you´re trying to do. — MakePeaceGreatAgain, Jun 26 '17 at 07:26
SettingKey = dbsetting.Applications.FirstOrDefault()?.Select(sk => sk.Settings.FirstOrDefault()?.SettingKey) — Laurent Lequenne, Jun 26 '17 at 07:28
I think select many is the thing you re looking for. https://stackoverflow.com/questions/958949/difference-between-select-and-selectmany — Drag and Drop, Jun 26 '17 at 07:38
@LaurentLequenne That won´t even compile, as `FirstOrDefault` returns an instance of `Application` which has no `Select`. — MakePeaceGreatAgain, Jun 26 '17 at 07:43
please add the Error/Exception as plain text and not as screenshot — Fabiano, Jun 26 '17 at 07:45
'dbsetting.Applications.FirstOrDefault()?.Settings.FirstOrDefault()?.SettingKey' should do the trick — Fabiano, Jun 26 '17 at 07:49

score 1 · Accepted Answer · answered Jun 26 '17 at 07:46

As you´re just interested in the one and only Application for every UserSetting I suppose you need this:

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

This will just return the very first (and probably only) Setting within the very first (and propbably also only) Application within every UserSetting. If any of the lists in between in empty (either Applications or Settings) the SettingKey will be null.

Querying a nested list using linq

1 Answers1