Adding a range of values to an ObservableCollection efficiently

Question

I have an ObservableCollection of items that is bound to a list control in my view.

I have a situation where I need to add a chunk of values to the start of the collection. Collection<T>.Insert documentation specifies each insert as an O(n) operation, and each insert also generates a CollectionChanged notification.

Therefore I would ideally like to insert the whole range of items in one move, meaning only one shuffle of the underlying list, and hopefully one CollectionChanged notification (presumably a "reset").

Collection<T> does not expose any method for doing this. List<T> has InsertRange(), but IList<T>, that Collection<T> exposes via its Items property does not.

Is there any way at all to do this?

Solution 1

The ObservableCollection exposes an protected Items property which is the underlying collection without the notification semantics. This means you can build a collection that does what you want by inheriting ObservableCollection:

class RangeEnabledObservableCollection<T> : ObservableCollection<T>
{
    public void InsertRange(IEnumerable<T> items) 
    {
        this.CheckReentrancy();
        foreach(var item in items)
            this.Items.Add(item);
        this.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
    }
}

Usage:

void Main()
{
    var collection = new RangeEnabledObservableCollection<int>();
    collection.CollectionChanged += (s,e) => Console.WriteLine("Collection changed");
    collection.InsertRange(Enumerable.Range(0,100));
    Console.WriteLine("Collection contains {0} items.", collection.Count);  
}

Solution 2

To make the above answer useful w/o deriving a new base class using reflection, here's an example:

public static void InsertRange<T>(this ObservableCollection<T> collection, IEnumerable<T> items)
{
  var enumerable = items as List<T> ?? items.ToList();
  if (collection == null || items == null || !enumerable.Any())
  {
    return;
  }

  Type type = collection.GetType();

  type.InvokeMember("CheckReentrancy", BindingFlags.Instance | BindingFlags.InvokeMethod | BindingFlags.NonPublic, null, collection, null);
  var itemsProp = type.BaseType.GetProperty("Items", BindingFlags.NonPublic | BindingFlags.FlattenHierarchy | BindingFlags.Instance);
  var privateItems = itemsProp.GetValue(collection) as IList<T>;
  foreach (var item in enumerable)
  {
    privateItems.Add(item);
  }

  type.InvokeMember("OnPropertyChanged", BindingFlags.Instance | BindingFlags.InvokeMethod | BindingFlags.NonPublic, null,
    collection, new object[] { new PropertyChangedEventArgs("Count") });

  type.InvokeMember("OnPropertyChanged", BindingFlags.Instance | BindingFlags.InvokeMethod | BindingFlags.NonPublic, null,
    collection, new object[] { new PropertyChangedEventArgs("Item[]") });

  type.InvokeMember("OnCollectionChanged", BindingFlags.Instance | BindingFlags.InvokeMethod | BindingFlags.NonPublic, null, 
    collection, new object[]{ new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset)});
}

Solution 3

This answer didn't show me the new entries in a DataGrid. This OnCollectionChanged works for me:

public class SilentObservableCollection<T> : ObservableCollection<T>
{
    public void AddRange(IEnumerable<T> enumerable)
    {
        CheckReentrancy();

        int startIndex = Count;

        foreach (var item in enumerable)
            Items.Add(item);

        OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, new List<T>(enumerable), startIndex));
        OnPropertyChanged(new PropertyChangedEventArgs("Count"));
        OnPropertyChanged(new PropertyChangedEventArgs("Item[]"));
    }
}