getting distinct value from a list of type comment

Question

    List<Comment> StreamItemComments = objStreamItem.GetComments();

...

    foreach (Comment Item in StreamItemComments)
        {
            if (ClientUser.UserName != Item.Sender)
            {
                Notification notificationObj = new Notification
                {
                    Sender = ClientUser.UserName,
                    Recipient = Item.Sender,
                    Value = "whatever value here",
                    TrackBack = "",
                    IsRead = false
                };
                notificationObj.Add();
            }
        }

What if there are two 'username' in List in Item.Sender. I'd like to send a notification once to the user. Here if there are duplicate usernames it will send two notifications because i am not filtering out duplicate Item.Senders from the list in StreamItemComments.

Answer 1

Consider writing a query to state your intentions. You want the distinct senders of the item comments, but only where the sender is not the client user. Sounds like a query, does it not?

var recipients = StreamItemComments
                    .Where(item => item.Sender != ClientUser.UserName)
                    .Select(item => item.Sender)
                    .Distinct();

You can then use this query to build your notifications

foreach (var item in recipients)
{
    var notificationObj = new Notification
    {
         Sender = ClientUser.UserName,
         Recipient = item,
         ...
    }

    notificationObj.Add();
}

You could also fit this object construction into the query, as well, but with your .Add() invocation on each object, I left it out of the query. It wouldn't be difficult to incorporate, although you'd still need to loop over the output and invoke .Add() for each result.

Answer 2

You can use a HashSet to determine if you already handled a username.

var set = new HashSet<string>();

foreach (var item in collection)
{
    if (set.Contains(item))
        continue;

    set.Add(item);

    // your notification code
}

For your concrete problem, set would contain usernames ( Item.Sender ). So you may want to change the Add() argument.

Answer 3

Use .Distinct() . Since you can't use the default comparer, you can implement one like this

class MyEqualityComparer : IEqualityComparer<Comment>
{
    public bool Equals(Comment x, Comment y)
    {
        return x.Sender.Equals(y.Sender);
    }

    public int GetHashCode(Comment obj)
    {
        return obj.Sender.GetHashCode();
    }
}

And then just filter them like this. You don't need the if statement.

List<Comment> StreamItemComments = objStreamItem.GetComments()
    .Distinct(new MyEqualityComparer())
    .Where(x => x.Sender != ClientUser.UserName)
    .ToList();

Answer 4

What you can do is in the

foreach ( Comment item in StreamItemComments)

add every notification to a Dictionary<user,msg>

and have another foreach key in Dictionary afterwards loop to send the actual message to user. This will ensure that only one message per user is sent out