Question

I want to filter my observable collection of viewmodel objects to the objects that are updated. I have subscribed to the Property Changed event for each viewmodel. But I am not sure how shall I track the objects so the end result would be only the objects that are updated in the UI.

    ProgramViewModel Cur=new ProgramViewModel(prg);
                        Cur.PropertyChanged += new PropertyChangedEventHandler(Cur_PropertyChanged);
                        program.Add(Cur);
//here program is my observable collection of viewmodels

 void Cur_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {  
          //What will be the code that will filter out only updated objects
            throw new NotImplementedException();
        }

//Now in my Save Event handler

private void Save_Click(object sender, RoutedEventArgs e)
        {
          foreach (ProgramViewModel model in program)
            {
                //I need only updated objects here to be added to database
            }
       }

Answer 1

I would just store them in a HashSet<ProgramViewModel>. That way, you won’t have to worry about tracking which ones are already in the collection:

private readonly HashSet<ProgramViewModel> changedPrograms = new HashSet<ProgramViewModel>();

void Cur_PropertyChanged(object sender, PropertyChangedEventArgs e)
{  
    changedPrograms.Add((ProgramViewModel)sender);
}

private void Save_Click(object sender, RoutedEventArgs e)
{
    foreach (ProgramViewModel model in changedPrograms)
    {
        ...
    }
}