A generic LongListCollection for use with the Silverlight Toolkit's LongListSelector

gistfile1.cs

public class LongListCollection<T, TKey> : List<LongListItem<T, TKey>>
{
    public LongListCollection()
    {
    }

    public LongListCollection(IEnumerable<T> items, Func<T, TKey> keySelector)
    {
        if (items == null)
            throw new ArgumentException("items");

        var groups = new Dictionary<TKey, LongListItem<T, TKey>>();

        foreach (var item in items)
        {
            var key = keySelector(item);

            if (groups.ContainsKey(key) == false)
                groups.Add(key, new LongListItem<T, TKey>(key));

            groups[key].Add(item);                
        }

        this.AddRange(groups.Values);
    }
}

public class LongListItem<T, TKey> : List<T>
{
    public LongListItem()
    {            
    }

    public LongListItem(TKey key)
    {
        this.Key = key;
    }

    public TKey Key
    {
        get;
        set;
    }

    public bool HasItems
    {
        get
        {
            return Count > 0;
        }
    }
}