c# mvvm nested ObservableCollection CollectionChanged NewStartingIndex >0

Question

I'm new to stackoverflow. I've been looking for a solution all day long so I decided to ask here.

I'm working on an application which uses WPF and databinding/MVVM. I have a nested ObservableCollection which is part of a Class Program. Both implement INotifyPropertyChange correctly.

In my constructor I create an ObservableColletion of type Program named ListPrograms and fill it from my data service with a list of program. That works all well. Also the binding is working correctly, in my master view I select a program and the nested list of prices is being displayed in the detail view. OnPropertyChange fires correctly for each property of a SalesPrice.

My requirement is that on user input the program needs to recalculate a cummulated price in every line item of the nested list. I have a method which does this correctly.

Im order to run that method I tried to attach a collectionchanged event.

`void GetPrograms()
    {        
        ListPrograms.Clear();          

        var prgms = DataService.GetProgramList(SearchStringProgram);

        ListPrograms=new ObservableCollection<Program>(prgms);

        foreach (var prg in ListPrograms)
        {
            prg.SalesPrices.CollectionChanged+=SalesPrices_CollectionChanged;
        }  `

///

   void SalesPrices_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {


        if (e.NewItems != null)
            foreach (SalesPrice item in e.NewItems)
                item.PropertyChanged += SalePrice_PropertyChanged;

        if (e.OldItems != null)
            foreach (SalesPrice item in e.OldItems)
               item.PropertyChanged -= SalePrice_PropertyChanged;
    }

    void SalePrice_PropertyChanged(object sender, PropertyChangedEventArgs e)
   {
        RecalcSalePrice(CurrentProgram.SalesPrices);
    }

The problem I have is that the collection change event is only registered for new SalesPrices I add to the collections at run time. I check the e.NewStartingIndex and it starts with the number of intial SalesPrice records. (eg. the first program and 9 sales prices it its ObservableColletion and the e.NewStartingIndex has a value of 8). When I add a new record into the list at runtime the even is registering and changes are notified.

I think this is happening because I'm passing the Program object from the dataservice with a filled list of SalesPrices and the event is only registering for new items.

Any idea?

Frank

Answer 1

Just perform the event subscription for all existing collection items when you first parse them:

void GetPrograms()
{        
    var prgms = DataService.GetProgramList(SearchStringProgram);

    ListPrograms=new ObservableCollection<Program>(prgms);

    foreach (var prg in ListPrograms)
    {
        prg.SalesPrices.CollectionChanged += SalesPrices_CollectionChanged;

        foreach (SalesPrice item in prg.SalesPrices)
        {
            item.PropertyChanged += SalePrice_PropertyChanged;
        }
    }
}