Order.cs

public class Order : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    // *** M E M B E R S ***
    private ObservableCollection<ProductBlock> _productBlocks;

    // *** P R O P E R T I E S ***
    public int Total { get; set; }
    public Customer Customer { get; set; }

    public ObservableCollection<ProductBlock> ProductBlocks
    {
        get { return _productBlocks; }
        set { _productBlocks = value; }
    }

    // *** C O N S T R U C T O R ***
    public Order()
    {
        _productBlocks = new ObservableCollection<ProductBlock>();
        _productBlocks.CollectionChanged += _productBlocks_CollectionChanged;
    }

    // Update the Total, if new blocks are added
    private void _productBlocks_CollectionChanged(object sender,
      System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        Total = 0;

        foreach (ProductBlock block in _productBlocks)
        {
            Total += block.BlockSum;
        }
    }
}