public ViewModel()
    {
        if (MyStrings == null)
        {
            MyStrings = new ObservableCollection <string>();
        }

        for (int i = 0; i < 5; i++)
        {
            MyStrings.Add("string " + i);
        }

        AddStringCommand = new AddLabelCommand(AddString);
    }
 public void AddString()
 {
     MyStrings.Add("string " + (MyStrings.Count));
 }