CComboBox class in MFC is used for manipulating the combo box. MFC allows achieving any custom behaviors on controls by inheriting from its own classes. This article illustrates an  owner-draw combo box class, which inherits from the CComboBox class.

Abstract class for implementing CComboBox owner draw:

   The abstract class CDrawC contains methods that are called from the combo box class only. They are

       virtual void DrawItem(LPDRAWITEMSTRUCT lpdis, bool bHasFocus) = 0;

   This DrawItem function actually draws the items inside the mfc combo boxes using the Win32 DRAWITEMSTRUCT.

      virtual void MeasureItem(LPMEASUREITEMSTRUCT lpmis) = 0;

    This function sets the item size once or every time depending of the owner-draw style: fixed or variable on the CComboBox derived combo box class.

      virtual void InitCombo(CComboBox* pCombo) = 0;

   Initializes the control holding the class if it is a CComboBox (or derived from it)

      virtual int AddItemCmb(CComboBox* pCombo, UINT nItem) = 0;

   Adds an item to the control holding the class if it is a CComboBox (or derived from it).

Using the CcomboBox derived owner draw class:

1. This class is attached to the combo box via it?s method SetDrawC. It must be added before the control is created, so it is better called in the parent dialog class constructor.

2. The Init() method of the control should be called immediately after it?s creation.

3. Items are added with the AddItem function of the control.

4. Combo boxes using from this type can be created either dynamically or using the dialog editor and after adding a variable of type CComboBox just rename it with CODrawCombo.

5. Don?t forget to call SetDrawC and Init.

6. Combo boxes must be created with the following styles: CBS_DROPDOWNLIST, CBS_HASSTRINGS, CBS_OWNERDRAWFIXED or CBS_OWNERDRAWVARIABLE.

   In this particular example a combo box listing colors is implemented. The init method is obsolete for there is no initialization needed for this case. The AddItem implementation inserts a not used string in the control and sets the item data to the color needed: 

virtual int AddItemCmb(CComboBox* pCombo, UINT nItem) {
            int n;
            n = pCombo->AddString("nu");
            pCombo->SetItemData(n, nItem);
            return n;
}

   After that in the drawing function the color is received as item data and drawn in a black rectangle like this:

CPen pen(PS_SOLID, 1, RGB(0,0,0));
CBrush brush;
?.
dc.SelectObject(pen);
dc.SelectObject(brush);
dc.Rectangle(ColRect);
?.
//lpszText is the number
dc.DrawText(lpszText, strlen(lpszText), TextRect, DT_SINGLELINE|DT_VCENTER);

   When the selection in the combo box is changed an event is raised. We catch this event to update the sample text with Invalidate. But the function that actually changes the color of the sample text is OnCtlColor. There you can see how to get the currently selected color from the control ie:

m_colorCombo.GetItemData(m_colorCombo.GetCurSel())

   Read more on general MFC Owner draw issues. This might help in reducing the general complexities of programming owner draw controls.