Question

I want a custom observablecollection so that I can catch when an item is added of type MyObj and call MyObj.DoCalc() on it before it’s added to Items. I also want to be able to pass in a list to myCustomCollection’s constructor, just like the base class, so that it sets items to that list.

public class MyObservableCollection : ObservableCollection<MyObj>
{

    protected override void OnCollectionChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add)
        {

            // perform calculation
        }
        base.OnCollectionChanged(e);

    }


}

That’s as far as I got. I’m not sure how to override the constructor and Items is readonly, so I don’t know how to set that to the passed in list. Thanks in advance.

Answer 1

For your first question, NotifyCollectionChangedEventArgs has an NewItems property. Just loop through that.

if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add)`
{
    foreach(var item in e.NewItems.Cast<MyObj>())
       item.DoCalc();
}

I’m not sure which order this is called.

For your 2nd question, here is the code you need:

public MyObservableCollection(IEnumerable<MyObj> items) : base(items)
{
}