Question

I have a converter that accepts an ObservableCollection as a parameter, and I’d like to re-evaluate it whenever a specific property on any item in the collection changes

For example: lets say I have bound a label to a collection of Person objects with a converter. The job of the converter is to count the number of Persons in the list that are female, and return “valid” for 1 female or “accepted” for 2. I’d like the converter to get called again anytime the Gender property on any Person object gets changed.

How can I accomplish this?

Answer 1

That’s a classic problem you end up having if you play around WPF long enough.

I’ve tried various solutions, but the one that works best is to use a BindingList like so:

public class WorldViewModel : INotifyPropertyChanged
{
   private BindingList<Person> m_People;
   public BindingList<Person> People
   {
      get { return m_People; }
      set
      {
         if(value != m_People)
         {
            m_People = value;
            if(m_People != null)
            {
               m_People.ListChanged += delegate(object sender, ListChangedEventArgs args)
               {
                  OnPeopleListChanged(this);
               };
            }
            RaisePropertyChanged("People");
         }
      }
   }

   private static void OnPeopleListChanged(WorldViewModel vm)
   {
      vm.RaisePropertyChanged("People");
   }

   public event PropertyChangedEventHandler PropertyChanged;
   void RaisePropertyChanged(String prop)
   {
      PropertyChangedEventHandler handler = this.PropertyChanged;
      if (handler != null)
      {
          handler(this, new PropertyChangedEventArgs(prop));
      }
   }
}

Then just bind to the People collection like you would do with an ObservableCollection, except bindings will be re-evaluated when any property in its items change.

Also, please note that OnPeopleListChanged is static, so no memory leaks.

And Person should implement INotifyPropertyChanged.