這個問題困擾了很長時間，沒有找到很好的解決方案；
方式1：在IDE界面上手動調整，這個很簡單，問題是如果代碼動態創建的CComboBox則無法用這個方法解決；
方式2：MoveWindow
方式3：SetWindowPos
方式2、3可以實現上面的需求，只是沒有辦法準確控制顯示的條數。終於在網上發現了http://hi.baidu.com/netspirit/blog/item/c033012cbb44a9e48b1399e0.html，同樣，這裏記錄下來，以免忘記。
void set_DropDownSize(CComboBox& box, UINT LinesToDisplay)
/*--------------------------------------------------------------------------
* Purpose: Set the proper number of lines in a drop-down list or
* combo box.
* Description: Resizes the combo box window to fit the proper number
* of lines. The window must exist before calling this function.
* This function should be called when the combo box is created, and when
* the font of the combo box changes. (e.g. WM_SETTINGCHANGE)
* Testing needed:
* Are there cases where SM_CYBORDER should be used instead of SM_CYEDGE?
* owner-draw variable height combo box
* Subclassed combo box with horizontal scroll-bar
* Returns: nothing
* Author: KTM
*--------------------------------------------------------------------------*/
{
ASSERT(IsWindow(box)); // Window must exist or SetWindowPos won't work
CRect cbSize; // current size of combo box
int Height; // new height for drop-down portion of combo box
box.GetClientRect(cbSize);
Height = box.GetItemHeight(-1); // start with size of the edit-box portion
Height += box.GetItemHeight(0) * LinesToDisplay; // add height of lines of text
// Note: The use of SM_CYEDGE assumes that we're using Windows '95
// Now add on the height of the border of the edit box
Height += GetSystemMetrics(SM_CYEDGE) * 2; // top & bottom edges
// The height of the border of the drop-down box
Height += GetSystemMetrics(SM_CYEDGE) * 2; // top & bottom edges
// now set the size of the window
box.SetWindowPos(NULL, // not relative to any other windows
0, 0, // TopLeft corner doesn't change
cbSize.right, Height, // existing width, new height
SWP_NOMOVE | SWP_NOZORDER // don't move box or change z-ordering.
);
}
需要注意的有兩點：
1、等填充完CComboBox後在調用上面的方法
2、CComboBox調用Create時，記得帶上WS_VSCROLL樣式，否則顯示的條數只有設定的幾條。當然這個問題並不是調用上面的方法引起的；