Filter a collection with another list using lambda expression

Question

I have 2 lists, the first one is an event list the second a list of id countries.

The event list contains a list of countries, so I am trying to filter the events that contain the countries that are sent in the parameter (list of countries).

I usually use foreachs for these cases, but I want to know if there is a way to filter these elements using lambda expressions?

This is my code using foreach

List<Event> finalList = new List<Event>();
 
 foreach (var eventItem in eventList)
    {
      foreach (var cItem in eventItem.CountrieList)
      {
        foreach (var pItem in countriesListParameter)
         {
           if (cItem .Id == pItem )
             {
                finalList.Add(eventItem )
             }

         }
      }

    }

Answer 1

With LINQ it looks so:

List<Event> finalEventList = eventList
    .Where(ev => ev.CountryList.Select(c => c.Id).Intersect(countriesListParameter).Any())
    .ToList();

So all events that have countries which Id is contained in the parameter-list.

You could also use Contains , but it is less efficient than Intersect(..).Any() if the lists are large:

List<Event> finalEventList = eventList
   .Where(ev => ev.CountryList.Any(c => countriesListParameter.Contains(c.Id)))
   .ToList();