使用linq从列表中的2个可能字段中选择distinct int

Question

I've looked for a solution but I can't quite figure it. It strikes me that linq should be able to handle this neatly but I'm not the best at formulating linq queries. Basically I have a class;

public class Message
{
    public int CreatedByPersonID { get; set; }
    public int? PostedToPersonID { get; set; }
}

What I'm looking for is the linq equivilant of;

List<int> personIDs = new List<int>();
foreach (Message message in messages)
{
    if (!personIDs.Contains(message.CreatedByPersonID))
    {
        personIDs.Add(message.CreatedByPersonID);
    }
    if (message.PostedToPersonID != null && !personIDs.Contains(message.PostedToPersonID.Value))
    {
        personIDs.Add(message.PostedToPersonID.Value);
    }
}

Is there a linq version of this? Or am I expecting too much of the technology...? I know I'm looking for some version of Select() but I don't know how to formulate a distinct with multiple fields

Answer 1

List<int> personIDs = 
     messages.SelectMany(m => m.PostedToPersonID.HasValue ?
                  new int[] { m.PostedToPersonID.Value, m.CreatedByPersonID } :
                  new int[] { m.CreatedByPersonID })
             .Distinct()
             .ToList();

Another option is enumerating twice:

List<int> personIDs =
    messages.Where(m => m.PostedToPersonID.HasValue)
            .Select(m => m.PostedToPersonID.Value)
            .Concat(messages.Select(m => m.CreatedByPersonID))
            .Distinct()
            .ToList();

But possibly without Linq you have more readable and faster code

public IEnumerable<int> GetAllPersonIdsFrom(IEnumerable<Message> messages)
{
     foreach(var message in messages)
     {
         yield return message.CreatedByPersonID;

         if (message.PostedToPersonID.HasValue)
             yield return message.PostedToPersonID.Value;
     }
}

---

One more solution - single enumerating with distinct values

public static List<int> GetDistinctPersonIdsFrom(IEnumerable<Message> messages)
{
    HashSet<int> ids = new HashSet<int>();

    foreach (var message in messages)
    {
        ids.Add(message.CreatedByPersonID);
        if (message.PostedToPersonID.HasValue)
            ids.Add(message.PostedToPersonID.Value);
    }

    return ids.ToList();
}