Combo Box Initialization

One of the more useless features of MFC is the ability to add items to a combo box in the resource editor. You might ask "Why?" Clearly it makes life easy. Well, it doesn't. In fact, it can make life impossible. For example, the only condition under which this is useful is if the strings are language-independent text and the whole ComboBox is completely insensitive to being sorted or unsorted. I've seen things like the resource editor adding items like

Black
Blue
Red
Green

and having code that says

switch(((CComboBox *)GetDlgItem(IDC_COLORS))->GetCurSel())
{
    case 0: // black
        color = RGB(0, 0, 0);
        break;
    case 1: // blue
        color = RGB(0, 0, 255);
        break;
}

You can see immediately that this is impossible to maintain; a change in the combo box resource has to be reflected in some unknown and unknowable code. Well, another solution is

#define COLOR_BLACK 0
#define COLOR_BLUE  1
...
switch(((CComboBox *)GetDlgItem(IDC_COLORS))->GetCurSel())
{
    case COLOR_BLACK:
}

This is merely syntactic aspartame (not even syntactic sugar) on a bad idea; it changes the problem not one bit. Another solution is to do something like

CString s;
((CComboBox *)GetDlgItem(IDC_COLORS))->GetLBText(s, 
              ((CComboBox*)GetDlgItem(IDC_COLORS)->GetCurSel());
if (s == CString("Black"))
{
    color = RGB(0, 0, 0);
}
else if (s == CString("Blue"))
{
    color = RGB(0, 0, 255);
}

etc. This has the advantage that at least you are not position-sensitive; but you are language-sensitive. Consider a European distributor who can edit the resource file, and change the strings:

Schwartz
Blau
Rot
Grün

The code fails, for the same reason. If the combo box is sorted, the order is all wrong; if the string compare is used, no color ever matches. An application that uses a combo box should be completely independent of the sort order and the language. Trust me. Been there, done that. You will only regret it.

Essentially, you must never write a ComboBox or a ListBox in which there is any assumption whatsoever about the meaning of a particular offset. The integers returned from GetCurSel are fundamentally meaningless except for retrieving the string data or ItemData of the control. They have no other significance, and to assign other significance to them is poor programming practice.

I have a class called "CIDCombo" which I use in all such cases. This was invented after the second time I did myself in using the preloaded combo box (Note: just because something is available, it does not mean that it is a good idea to use it!) What CIDCombo does is allow me to specify a pair, a string ID from the string table and a relevant mapped value, in a table. The table is basically

typedef struct IDData {
    UINT id;
    DWORD value;
}; // defined in IDCombo.h file

IDData colors [] = {
    {IDS_BLACK, RGB(  0,   0,   0)},
    {IDS_BLUE,  RGB(  0,   0, 255)},
    ...
    {0, 0} // end of table
};<

the core loop is essentially

void IDCombo::load(IDData * data )
{
    for(int i = 0; data[i].id != 0; i++)
    {
        CString s;
        s.LoadString(data[i].id);
        int index = AddString(s);
        SetItemData(index, data[i].value);
    }
}

So what happens in my OnInitDialog is:

BOOL CMyDialog::OnInitDialog()
{
    ...
    c_Colors.load(colors); // Note: <A href="http://www.codeproject.com/cpp/NoGetDlgItem.asp">no GetDlgItem</A>, ever!
    ...
}

This has numerous advantages over the initialize-in-resource method:

1. It has a single point of definition for all values in the combo box.
2. There is only one place a change needs to be made to add or delete items
3. It is intrinsically insensitive to sorting issues
4. It is language-independent.

If the string IDS_BLACK is changed to "Noir" or "Schwartz" or something else, the color value is always RGB(0,0,0). And if the combo box was sorted, or not sorted, it doesn't matter; the color names are always properly matched to their color values. Or control flow settings. Or data bits values. Or whatever. Essentially, an combo box that could be initialized from the resource is better served by this method. I've never found an exception.

The class is available on the CD-ROM that accompanies our book (Win32 Programming, Rector & Newcomer, Addison-Wesley, 1997), and an instance of it can be downloaded free from this Web site

Another cool feature of CIDCombo is that it automatically resizes the dropdown list so that if at all possible, all the items always show without a scrollbar. No more silly resizing the dropdown "by hand" in the hopes that everything will fit! You'll always see everything, scrollbar-free, unless the whole selection won't fit in the window. The height of the dropdown is dynamically adjusted to fit as many items as possible in, given the position of the combo box on the screen (it will pop up above the combo box if it needs more space and the combo box is low on the screen).

What makes this really nice is that whenever you want the actual value, you can simply use GetItemData to obtain the value.

COLORREF CMyDialog::getColor()
{
    int sel = c_Colors.GetCurSel();
    if(sel == CB_ERR)
        return RGB(0, 0, 0); // or other suitable default value
    return c_Colors.GetItemData(sel);
}

How do you handle more complex information? Well one way is to define a struct for the group of information, for example, a somewhat silly example is a dropdown list that describes bean types and their packers. (The reason it is silly is that this would actually be done from a database, but the idea is to make a simple sample)

typedef struct {
    UINT weight;
    UINT company;
} BeanDescriptor, *LPBeanDescriptor;

BeanDescriptor kidney = { 16, IDS_REDPACK};
LineDescriptor vegveg = { 12, IDS_HEINZ};
LineDescriptor green =  { 14, IDS_GENERIC};

IDData lines [] = {
    { IDS_KIDNEY, (DWORD)&kidney},
    { IDS_VEGETARIAN, (DWORD)&vegveg},
    { IDS_GREENBLOCKS, (DWORD)&green},
    { 0, 0} // EOT
};

To use the data, you need to do the following:

LPBeanDescriptor getBean()
{
    int sel = c_Beans.GetCurSel();
    if(sel == CB_ERR)
        return NULL;
    return (LPBeanDescriptor)c_Beans.GetItemDataPtr(sel);
}

How do you select an item? Well, you need the moral equivalent of FindStringExact. In this case, the selection is based on an ItemData comparison. For example:

int CIDCombo::Select(DWORD value)
{
    for(int i = 0; i < CComboBox::GetCount(); i++)
    { /* compare */
        DWORD v = CComboBox::GetItemData(i);
        if(value == v)
        { /* found it */
            CComboBox::SetCurSel(i);
            return i;
        } /* found it */

        CComboBox::SetCurSel(-1);
        return CB_ERR;
    }
}

You can get my implementation of CIDCombo by clicking the link at the top of the article.

The views expressed in these essays are those of the author, and in no way represent, nor are they endorsed by, Microsoft.

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