How to resize ObservableCollection?

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);
            }
        }
    }
}

score 8 · Accepted Answer · answered Nov 29 '10 at 16:20

8

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

answered Nov 29 '10 at 16:20

Simon Mourier

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Is it trivial to do so? I was hoping for an answer and not another question :) – Jeff LaFay Nov 29 '10 at 16:22
Your idea worked, just wish you would have elaborated more. Either way, I'm adding my solution to my question. Thanks for the help. – Jeff LaFay Nov 29 '10 at 17:03

score 7 · Answer 2 · answered Nov 29 '10 at 17:39

7

try this instead:

public class FixedSizeObservableCollection<T> : ObservableCollection<T>
{
    private readonly int maxSize;
    public FixedSizeObservableCollection(int maxSize)
    {
        this.maxSize = maxSize;
    }

    protected override void InsertItem(int index, T item)
    {
        if (Count == maxSize)
            return; // or throw exception
        base.InsertItem(index, item);
    }
}

answered Nov 29 '10 at 17:39

Dean Chalk

20,076
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1

Thats pretty much what I ended up going with myself as suggested by Simon. – Jeff LaFay Nov 29 '10 at 18:03

score 3 · Answer 3 · answered Nov 29 '10 at 16:31

3

You can roll your own collection class that implements INotifyCollectionChanged and INotifyPropertyChanged. You can do it fairly easily by using ObservableCollection internally in your own collection class.

answered Nov 29 '10 at 16:31

Jakob Christensen

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How to resize ObservableCollection?

3 Answers3