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Binding to ObservableCollection.Count doesn't update when OC is updated
<toolkit:NumericUpDown x:Name="numberOfCubesUpDown" IsEnabled="True" Maximum="9" Minimum="1" Style="{StaticResource ButtonSpinnerHorizontalStyle}" Value="{Binding Path=Cubes.Count}" Height="30" FontSize="14">
  <i:Interaction.Triggers>
     <i:EventTrigger EventName="ValueChanged">
        <cmd:EventToCommand Command="{Binding ChangeNumberOfCubesCommand}" PassEventArgsToCommand="True"></cmd:EventToCommand>
     </i:EventTrigger>
  </i:Interaction.Triggers>
</toolkit:NumericUpDown>

public CubeSet Cubes
    {
        get { return _cubes; }

        set
        {
            if (_cubes == value) { return; }
            _cubes = value;
        }
    }

Cubes = new CubeSet();
        for (int i = 0; i < 6; i++) { Cubes.Add(new Cube()); }

using System.ComponentModel;

namespace Sample.ViewModels
{
    public class ViewModelBase : INotifyPropertyChanged
    {
        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        #endregion

        protected void OnPropertyChanged(string name)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(name));
            }
        }
    }
}

public CubeSet Cubes
{
    get { return _cubes; }

    set
    {
        if (_cubes == value) { return; }
        _cubes = value;

        OnPropertyChanged("Cubes");
    }
}