Combo boxes

A combo box consists of a list and a selection field. The list presents the options that a user can select and the selection field displays the current selection. Except in drop-down list boxes, the selection field is an edit control and can be used to enter text not available in the list.

Combo boxes can be characterized by type and style. Combo box types determine whether the combo box list is a drop-down list and whether the selection field is an edit control. A drop-down list appears only when the user opens it, so it uses less screen space than a list that is always visible. If the selection field is an edit control, the user can enter information not available in the list; otherwise, the user can only select items in the list.

The following table shows the three combo box types and indicates their drop-down list and edit control attributes.

Combo box styles define specific properties of a combo box. You can combine styles; however some styles only apply to certain combo box types.

Styles

• No height adjust.Specifies that the size of the combo box is exactly the size specified by the application when it created the combo box. Normally, the system sizes a combo box so that it does not display partial items.
• Local memory. Specifies that an owner-drawn combo box contains items consisting of strings. The combo box maintains the memory and address for the strings so the application can use the CB_GETLBTEXT message to retrieve the text for a particular item.
• DOS char convertion.Converts text entered in the combo box edit control from the Windows character set to the OEM character set and then back to the Windows character set. This ensures proper character conversion when the application calls the CharToOem function to convert a Windows string in the combo box to OEM characters. This style is most useful for combo boxes that contain file names and applies only to combo boxes created with the CBS_SIMPLE or CBS_DROPDOWN style.
• Owner draw fixed.Specifies that the owner of the list box is responsible for drawing its contents and that the items in the list box are all the same height. The owner window receives a WM_MEASUREITEM message when the combo box is created and a WM_DRAWITEM message when a visual aspect of the combo box has changed.
• Owner draw variable.Specifies that the owner of the list box is responsible for drawing its contents and that the items in the list box are variable in height. The owner window receives a WM_MEASUREITEM message for each item in the combo box when you create the combo box and a WM_DRAWITEM message when a visual aspect of the combo box has changed.
• Auto hscroll. Automatically scrolls the text in an edit control to the right when the user types a character at the end of the line. If this style is not set, only text that fits within the rectangular boundary is allowed.