C#

public bool IsEmpty(DataSet ds)
{
  if (ds == null) throw new ArgumentNullException("ds");
  foreach (DataTable dt in ds.Tables)
    if (dt.Rows.Count > 0) return false;
  return true;
}

private DateTime[] annuallyBoldedDates;
private DateTime[] boldedDates;
public DateTime[] AnnuallyBoldedDates
{
  get { return annuallyBoldedDates; }
  set { annuallyBoldedDates = value; }
}
public DateTime[] BoldedDates
{
  get { return boldedDates; }
  set { boldedDates = value; }
}

protected override void OnDropDown(EventArgs e)
{
  base.OnDropDown(e);
  MonthCalendar mc = GetMonthCalendar();
  if (mc != null)
  {
    mc.AnnuallyBoldedDates = annuallyBoldedDates;
    mc.BoldedDates = boldedDates;
  }
}

[DllImport("user32.dll")]
private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam);
private const int DTM_GETMONTHCAL = 0x1008;
private MonthCalendar GetMonthCalendar()
{
  IntPtr hWnd = SendMessage(this.Handle, DTM_GETMONTHCAL, IntPtr.Zero, IntPtr.Zero);
  if (hWnd != IntPtr.Zero)
    return Control.FromHandle(hWnd) as MonthCalendar;
}