Need LINQ expression to filter table based on foreign key values

Question

I have a table of WorldEvents. Each WorldEvent has a list of Presentations, that happened in some country, regarding that WorldEvent

public class WorldEvent
{
    public int ID { get; set; }
    public string Name { get; set; }
    public List<Presentation> PresentationList { get; set; }
}

public class Presentation
{
    public int ID { get; set; }
    public string Name { get; set; }
    public string Country { get; set; }
}

public class WorldEventService
{
    public List<WorldEvent> GetWorldEvents()
    {
        List<WorldEvent> worldEventList = new List<WorldEvent>();
        List<Presentation> presentationList = new List<Presentation>();

        // Create list of Presentations for WorldEvent_1
        presentationList = new List<Presentation>()
        {
            new Presentation() { ID = 1, Name = "Presentation_1", Country = "Germany",},
            new Presentation() { ID = 2, Name = "Presentation_2", Country = "UK",},
            new Presentation() { ID = 3, Name = "Presentation_3", Country = "UK",},
        };

        // Add WorldEvent_1 to the list of WorldEvents
        worldEventList.Add(new WorldEvent()
        {
            ID = 1,
            Name = "WorldEvent_1",
            PresentationList = presentationList,
        });

        // Create list of Presentations for WorldEvent_2
        presentationList = new List<Presentation>()
        {
            new Presentation() { ID = 4, Name = "Presentation_4", Country = "USA",},
            new Presentation() { ID = 5, Name = "Presentation_5", Country = "UK",},
            new Presentation() { ID = 6, Name = "Presentation_6", Country = "Japan",},
        };

        // Add WorldEvent_2 to the list of WorldEvents
        worldEventList.Add(new WorldEvent()
        {
            ID = 2,
            Name = "WorldEvent_2",
            PresentationList = presentationList,
        });

        // Create list of Presentations for WorldEvent_3
        presentationList = new List<Presentation>()
        {
            new Presentation() { ID = 7, Name = "Presentation_7", Country = "France",},
            new Presentation() { ID = 8, Name = "Presentation_8", Country = "Germany",},
            new Presentation() { ID = 9, Name = "Presentation_9", Country = "Japan",},
        };

        // Add WorldEvent_3 to the list of WorldEvents
        worldEventList.Add(new WorldEvent()
        {
            ID = 3,
            Name = "WorldEvent_3",
            PresentationList = presentationList,
        });

        return worldEventList;
    }
}

Now - how can I get a list of WorldEvents, whose Presentations took place in the UK. And - in the list of my interest, WorldEvents should contain info about those UK Presentations only. In other word, I need this as result:

1. WorldEvent_1(Presentation_2, Presentation_3)
2. WorldEvent_2(Presentation_5)

Answer 1

If I've understood what you want. There are many ways to do this, however you can filter first, then recreate your WorldEvents with the filtered list of Presentation

var country = "UK";

var result = worldEventList.Where(x => x.PresentationList.Any(y => y.Country == country))
                           .Select(x => new WorldEvent()
                               {
                                  ID = x.ID,
                                  Name = x.Name,
                                  PresentationList = x.PresentationList
                                                      .Where(y => y.Country == country)
                                                      .ToList()
                                }).ToList();

or as noted by Gert Arnold in the comments you could filter after the fact

var result = worldEventList.Select(x => new WorldEvent()
                 {
                     ID = x.ID,
                     Name = x.Name,
                     PresentationList = x.PresentationList
                                         .Where(y => y.Country == country).ToList()
                 }).Where(x => x.PresentationList.Any())
                   .ToList();

_{Note : Because this is not projecting (selecting) each Presentation, any change you make to a Presentation in the result will be reflected in the original data. If you don't want this, you will need to recreate each Presentation}

Answer 2

var worldEvent = new WorldEventService.GetWorldEvents();

var filter = "";//userInput

var filteredResult = worldEvent.Select(r => new WorldEvent
                     { 
                         PresentationList = r.PresentationList.Where(c => c.Country == filter).ToList(),
                         ID = r.Id,
                         Name = r.Name 
                     }).ToList();