can i send CBN_CLOSEUP message to COMBOBOX?

oh,
i have a problem,would you help me?
i create a dialog that includes a COMBOBOX ，a BUTTON, the COMBOBOX has no member,when i click the COMBOBOX's down-arrow,then i want to click the BUTTON,but it can not click,because the focus is in COMBOBOX(the COMBOBOX show its members,although it is no members),so how can i resolve the problem,

case WM_COMMAND:
switch ( LOWORD( wParam )) {
case IDC_USER:
{
switch(HIWORD(wParam) )
{
case CBN_DROPDOWN:
{
HWND hwndComboBox;
int n=0;
hwndComboBox = GetDlgItem( hwndDlg, IDC_USER );
n = SendMessage( hwndComboBox, CB_GETCOUNT, 0, 0 );
/*if (n == 0)*/
{
int t=(int)LOWORD(wParam);
HWND h=(HWND)(lParam);
SendMessage( hwndComboBox, CB_SHOWDROPDOWN, FALSE, 0 );
}
break;
}

SendMessage( hwndComboBox, CB_SHOWDROPDOWN, FALSE, 0 ); it seems invalidly,it is not hide.

i have resolved this problem,and handled WM_LBUTTONDOWN message in WindowProc.but also thank you.