ID Number: Q83302
3.00 3.10
WINDOWS
Summary:
Windows sends a WM_GETDLGCODE message to controls in a dialog box or
in a window where the IsDialogMessage function handles keyboard input.
Generally, an application processes the WM_GETDLGCODE message to
prevent Windows from performing default processing in response to
keyboard messages. The WM_KEYDOWN, WM_SYSCHAR, and WM_CHAR messages
are examples of keyboard messages.
This article discusses the various codes that make up the value
returned from the WM_GETDLGCODE message.
More Information:
Windows sends a WM_GETDLGCODE message to a control for the following
three reasons:
- To determine whether the control will process a particular type of
input.
- To determine whether the text contents of an edit control are
selected when, as a result of the user pressing the TAB key,
the edit control receives the input focus.
- To determine the type of a button.
The following text documents the values of the wParam and lParam
parameters for the WM_GETDLGCODE message:
Parameter Description
--------- -----------
wParam Not used.
lParam If lParam is not NULL, it is a far pointer to an MSG
structure that contains a message that is being sent to
the control. Windows versions 3.0 and 3.1 use lParam only
to send keyboard input to the control. Keyboard messages
include WM_KEYDOWN, WM_SYSCHAR, and WM_CHAR. Future
versions of Windows may use lParam to send other message
types to controls.
The window procedure for each predefined control returns an
appropriate value in response to a WM_GETDLGCODE message. The value is
one or more of the following codes, combined with the Boolean OR
operator:
Code Meaning
---- -------
DLGC_BUTTON Control is a button (of any kind).
DLGC_DEFPUSHBUTTON Control is a default push button.
DLGC_HASSETSEL Windows will send an EM_SETSEL message to the
control to select its contents.
DLGC_RADIOBUTTON Control is an option (radio) button.
DLGC_STATIC Control is a static control.
DLGC_UNDEFPUSHBUTTON Control is a push button but not the default
push button.
DLGC_WANTALLKEYS Control processes all keyboard input.
DLGC_WANTARROWS Control processes arrow keys.
DLGC_WANTCHARS Control processes WM_CHAR messages.
DLGC_WANTMESSAGE Control processes the message in the MSG
structure that lParam points to.
DLGC_WANTTAB Control processes the TAB key.
The return codes above can be used by user-defined controls or, in a
subclass procedure, to modify the behavior of predefined controls. To
subclass a control, call the predefined control's window procedure
first, then modify the necessary bits in the return code.
DLGC_WANTALLKEYS, DLGC_WANTARROWS,
DLGC_WANTCHARS, DLGC_WANTMESSAGE, and DLGC_WANTTAB Return Codes
---------------------------------------------------------------
When a control processes the WM_GETDLGCODE message and the value it
returns has one of the DLGC_WANT* bits set, the control will process
the specified message type and Windows will not do any default
processing for messages of the specified type.
For example, the code returned by a list box includes DLGC_WANTARROWS
to indicate that the list box processes arrow keys. When a list box
has the focus and the user presses a DOWN ARROW key, Windows sends a
WM_GETDLGCODE message to the list box. Because the return value
includes the DLGC_WANTARROWS code, Windows allows the list box to
process the arrow keystroke and performs no further processing. If the
return value did not include the DLGC_WANTARROWS code, Windows would
continue processing the arrow keystroke and would change the focus to
the next control in the current control group.
As another example, the value returned by an edit control includes the
DLGC_WANTCHARS code while the value returned by a button does not.
Consequently, if a button has the focus, and the user types a valid
mnemonic character, Windows sets the focus to the control in the
dialog box that corresponds to the mnemonic. (If a control has a
mnemonic character, it is underlined in the control's label.) If an
edit control has the focus and the user types a mnemonic character,
however, Windows does not change the input focus because the edit
control processes the resulting WM_CHAR message and Windows does not
perform its default processing for a mnemonic character.
DLGC_WANTMESSAGE Code
---------------------
A control returns a value that includes the DLGC_WANTMESSAGE code
after it processes the message sent through the lParam that
accompanies the WM_GETDLGCODE message. The DLGC_WANTMESSAGE code
indicates that the application does not want default processing for
the message to continue. The messages sent to the control include
WM_KEYDOWN, WM_SYSCHAR, and WM_CHAR. Future versions of Windows could
send other messages to controls using this mechanism.
The following code provides an example of processing the WM_GETDLGCODE
message in a control's subclass procedure. In the example, the user
presses the "X" key to select a check box and presses the "O" key to
clear the check box:
case WM_GETDLGCODE:
// Call the check box window procedure first
lRet = CallWindowProc(lpCheckProc, hWnd, wMessage, wParam,
lParam);
// If lParam points to an MSG structure
if (lParam)
{
lpmsg = (LPMSG)lParam;
if (lpmsg->message == WM_CHAR)
{
if (lpmsg->wParam == 'x' || lpmsg->wParam == 'X')
{
// Select the check box when user presses "X"
SendMessage(hWnd, BM_SETCHECK, TRUE, 0);
lRet |= DLGC_WANTMESSAGE;
}
else if (lpmsg->wParam == 'o' || lpmsg->wParam == 'O')
{
// Clear the check box when user presses "O"
SendMessage(hWnd, BM_SETCHECK, FALSE, 0);
lRet |= DLGC_WANTMESSAGE;
}
}
}
return lRet;
When a check box control's subclass procedure includes the code above,
Windows performs no further processing for WM_CHAR messages for the X,
x, O, and o characters because the value returned from WM_GETDLGCODE
includes the DLGC_WANTMESSAGE code. In the example above, the control
could have returned DLGC_WANTCHARS instead of DLGC_WANTMESSAGE because
the WM_CHAR message is the only message processed by the control.
DLGC_HASSETSEL Code
-------------------
An edit control returns a value that includes the DLGC_HASSETSEL code
to indicate that Windows should select all the text in an edit control
when the control receives the input focus through the tabbing
sequence.
For example, when a control in a dialog box receives the focus because
the user pressed the TAB key, Windows sends a WM_GETDLGCODE message to
the control. If the value returned from the edit control includes the
DLGC_HASSETSEL code, the edit control indicates that all text in the
edit control should be selected. Consequently, Windows sends an
EM_SETSEL message to the control to select all its contents.
An application can alter this behavior and prevent the contents from
being selected when the control receives the focus through tabbing, by
subclassing the edit control and removing the DLGC_HASSETSEL code from
its return value. Note that the subclassing code below does not change
any other bits in the return value.
// In the subclass procedure
case WM_GETDLGCODE:
// Call the original edit control window procedure
lRet = CallWindowProc(lpEditProc, hWnd, wMessage, wParam,
lParam);
// Clear the DLGC_HASSETSEL bit from the return value
lRet &= ~DLGC_HASSETSEL;
return lRet;
DLGC_BUTTON, DLGC_DEFPUSHBUTTON,
DLGC_UNDEFPUSHBUTTON, DLGC_RADIOBUTTON, DLGC_STATIC Codes
---------------------------------------------------------
These codes are used to determine a control's attributes.
Additional reference words: 3.00 3.10