A check box consists of a square box and application-defined text (label), an icon, or a bitmap, that indicates a choice the user can make by selecting the button. Applications typically display check boxes in a group box to permit the user to choose from a set of related, but independent options. For example, an application might present a group of check boxes from which the user can select error conditions that produce warning beeps.
A check box can be one of four styles: standard, automatic, three-state, and automatic three-state, as defined by the constants BS_CHECKBOX, BS_AUTOCHECKBOX, BS_3STATE, and BS_AUTO3STATE, respectively. Each style can assume two check states: checked (an check mark inside the box) or unchecked (no check mark). In addition, a three-state check box can assume an indeterminate state (a grayed box inside the check box). Repeatedly clicking a standard or automatic check box toggles it from checked to unchecked and back again. Repeatedly clicking a three-state check box toggles it from checked to unchecked to indeterminate and back again.
When the user clicks a check box (of any style), the check box receives the keyboard focus. The system sends the check box's parent window a WM_COMMAND message containing the BN_CLICKED notification code. The parent window doesn't acknowledge this message if it comes from an automatic check box or automatic three-state check box, because the system automatically sets the check state for those styles. But the parent window must acknowledge the message if it comes from a check box or three-state check box because the parent window is responsible for setting the check state for those styles. Regardless of the check box style, the system automatically repaints the check box once its state is changed.