Radio Buttons

A radio button consists of a round button and application-defined text (a label), an icon, or a bitmap that indicates a choice the user can make by selecting the button. An application typically uses radio buttons in a group box to permit the user to choose from a set of related, but mutually exclusive options. For example, the application might present a group of radio buttons from which the user can select a format preference for text selected in the client area. The user could select a left-aligned, right-aligned, or centered format by selecting the corresponding radio button. Typically, the user can select only one option at a time from a set of radio buttons.

A radio button can be one of two styles: standard or automatic, as defined by the constants BS_RADIOBUTTON and BS_AUTORADIOBUTTON. Each style can assume two check states: checked (a dot in the button) or unchecked (no dot in the button). Repeatedly selecting a radio button (standard or automatic) toggles it from checked to unchecked and back again.

When the user selects either state, the radio button receives the keyboard focus from Windows, which sends the button'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 radio button because Windows automatically sets the check state for that style. But the parent window should acknowledge the message if it comes from a radio button because the parent window, not Windows, is responsible for setting the check state 1for that style. Regardless of the radio button style, Windows automatically repaints the button as its state changes.

When the user selects an automatic radio button, Windows automatically sets the check state of all other radio buttons within the same group to unchecked. The same behavior is available for standard radio buttons by using the WS_GROUP style, as discussed in Dialog Boxes.