The list box of the combobox is not visible

void CDialog::DoDataExchange(CDataExchange* pDX)
{
 CDialog::DoDataExchange(pDX);

  DDX_Control(pDX, IDC_COMBO, m_Combo);
  DDX_CBIndex(pDX, IDC_COMBO, m_ComboValue);
}

BOOL CDialog::OnInitDialog()
{
 CDialog::OnInitDialog();

 m_Combo.ShowWindow(SW_SHOW);

 FromMMToDlg();

 return TRUE;
}

void CDialog::FromMMToDlg()
{
 m_Combo.AddString("aaaa");
 m_Combo.AddString("bbbb");

 OnSelchangeCombo();
}

BEGIN_MESSAGE_MAP(CDialog, CDialog)
  ON_CBN_SELCHANGE(IDC_COMBO, OnSelchangeCombo)
END_MESSAGE_MAP()

void CDialog::OnSelchangeCombo()
{
 UpdateData(TRUE);

 switch(m_ComboValue)
 {
 case 0:
  // Part A
  break;
 case 1:
  // Part B
  break;
 }
}