Question

Is it possible to resize an Observable Collection or perhaps restrict the max amount of collection items? I have an ObservableCollection as a property in a View Model (using the MVVM pattern).

The view binds to the collection and I’ve attempted to hack a solution by providing an event handler that executes when a CollectionChanged event occurs. In the event handler, I trimmed the collection by removing as many items off the top of the collection as necessary.

ObservableCollection<string> items = new ObservableCollection<string>();
items.CollectionChanged += new NotifyCollectionChangedEventHandler(Items_Changed);

void Items_Changed(object sender, NotifyCollectionChangedEventArgs e)
{
    if(items.Count > 10)
    {
        int trimCount = items.Count - 10;
        for(int i = 0; i < trimCount; i++)
        {
            items.Remove(items[0]);
        }
    }
}

This event handler yields an InvalidOperationException because it doesn’t like the fact that I alter the collection during a CollectionChanged event. What should I do to keep my collection sized appropriately?

Solution:
Simon Mourier asked if I could create a new collection derived from ObservableCollection<T> and override InsertItem() and thats just what I did to have an auto-resizing ObservableCollection type.

public class MyCollection<T> : ObservableCollection<T>
{
    public int MaxCollectionSize { get; set; }

    public MyCollection(int maxCollectionSize = 0) : base()
    {
        MaxCollectionSize = maxCollectionsize;
    }

    protected override void InsertItem(int index, T item)
    {
        base.InsertItem(index, item);

        if(MaxCollectionSize > 0 && MaxCollectionSize < Count)
        {
            int trimCount = Count - MaxCollectionSize;
            for(int i=0; i<trimCount; i++)
            {
                RemoveAt(0);
            }
        }
    }
}

Answer 1

Can you derive the ObservableCollection class and override the InsertItem method?