Question

Hard question to understand perhaps, but let me explain. I have a List of Channel-objects, that all have a ChannelId property (int). I also have a different List (int) – SelectedChannelIds, that contains a subset of the ChannelId-s.

I want to select (through LINQ?) all the Channel-objects that has a ChannelId-property matching one in the second List.

in other words, I have the following structure:

public class Lists
{
    public List<Channel> AllChannels = ChannelController.GetAllChannels();
    public List<int> SelectedChannelIds = ChannelController.GetSelectedChannels();

    public List<Channel> SelectedChannels; // = ?????
}

public class Channel
{
    // ...
    public int ChannelId { get; set; }
    // ...
}

Any ideas on what that LINQ query would look like? Or is there a more effective way? I’m coding for the Windows Phone 7, fyi.

Answer 1

You can use List.Contains in a Where clause:

public Lists()
{
    SelectedChannels = AllChannels
        .Where(channel => SelectedChannelIds.Contains(channel.ChannelId))
        .ToList();
}

Note that it would be more efficient if you used a HashSet<int> instead of a List<int> for the SelectedChannelIds. Changing to a HashSet will improve the performance from O(n²) to O(n), though if your list is always quite small this may not be a significant issue.