Hi,

This is expected behavior due to the setting the DefaultDialogResult to true. It should be set to false when editing an appointment. Please use the approach demonstrated below:

private void ScheduleView_ShowDialog(object sender, ShowDialogEventArgs e)
{
    var viewModel = e.DialogViewModel as AppointmentDialogViewModel;
    if (viewModel != null)
    {
        e.DefaultDialogResult = viewModel.ViewMode == AppointmentViewMode.Add;
    }

    e.Cancel = true;
    e.Handled = true;
}

Regards,
Kalin