// provides unlimited amount of attached properties on demand
class DependencyPropertyDepot : DependencyObject
{
    #region attached property int AllocatedPropertyCount
    public static int GetAllocatedPropertyCount(DependencyObject obj)
    {
        return (int)obj.GetValue(AllocatedPropertyCountProperty);
    }

    public static void SetAllocatedPropertyCount(DependencyObject obj, int value)
    {
        obj.SetValue(AllocatedPropertyCountProperty, value);
    }

    public static readonly DependencyProperty AllocatedPropertyCountProperty =
        DependencyProperty.RegisterAttached("AllocatedPropertyCount", typeof(int), typeof(DependencyPropertyDepot));
    #endregion

    static List<DependencyProperty> existingProperties = new List<DependencyProperty>();

    static DependencyProperty GetDP(int index)
    {
        EnsureCount(index + 1);
        return existingProperties[index];
    }

    static void EnsureCount(int count)
    {
        if (existingProperties.Count < count)
        {
            for (int i = existingProperties.Count; i < count; i++)
                GenerateNewProperty(i);
        }
    }

    static void GenerateNewProperty(int index)
    {
        Debug.Assert(existingProperties.Count == index);
        existingProperties.Add(DependencyProperty.RegisterAttached("Property" + index, typeof(object), typeof(DependencyPropertyDepot)));
    }

    public static DependencyProperty AllocateNewPropertyFor(DependencyObject obj)
    {
        var idx = GetAllocatedPropertyCount(obj);
        var dp = DependencyPropertyDepot.GetDP(idx);
        SetAllocatedPropertyCount(obj, idx + 1);
        return dp;
    }
}