private int GetNextItem(int index)
{
if (index < 0)
{
throw new ArgumentException("index");
}
int count = NativeMethods.SendMessage(this.Handle, NativeMethods.TB_BUTTONCOUNT, 0, 0);
int nextIndex = index;
do
{
nextIndex = (nextIndex + 1) % count;
}
while ((nextIndex != index) && (!items[nextIndex].IsVisible));
return nextIndex;
}