SetWindowLong
The SetWindowLong
function changes an attribute of the specified window. The function also sets a
32-bit (long) value at the specified offset into the extra window memory of a
window.
LONG SetWindowLong(
|
HWND hWnd, |
// handle of window |
|
int nIndex, |
// offset of value
to set |
|
LONG dwNewLong |
// new value |
|
); |
|
Parameters
hWnd
Identifies
the window and, indirectly, the class to which the window belongs.
nIndex
Specifies the
zero-based offset to the value to be set. Valid values are in the range zero
through the number of bytes of extra window memory, minus 4; for example, if
you specified 12 or more bytes of extra memory, a value of 8 would be an index
to the third 32-bit integer. To set any other value, specify one of the
following values:
|
Value |
Action |
|
GWL_EXSTYLE |
Sets a new
extended window style. |
|
GWL_STYLE |
Sets a new
window style. |
|
GWL_WNDPROC |
Sets a new
address for the window procedure. |
|
GWL_HINSTANCE |
Sets a new
application instance handle. |
|
GWL_ID |
Sets a new
identifier of the window. |
|
GWL_USERDATA |
Sets the
32-bit value associated with the window. Each window has a corresponding
32-bit value intended for use by the application that created the window. |
The following
values are also available when the hWnd parameter identifies a dialog
box:
|
Value |
Action |
|
DWL_DLGPROC |
Sets the
new address of the dialog box procedure. |
|
DWL_MSGRESULT |
Sets the
return value of a message processed in the dialog box procedure. |
|
DWL_USER |
Sets new
extra information that is private to the application, such as handles or
pointers. |
dwNewLong
Specifies the
replacement value.
Return Values
If the
function succeeds, the return value is the previous value of the specified
32-bit integer.
If the
function fails, the return value is zero. To get extended error information,
call GetLastError.
If the
previous value of the specified 32-bit integer is zero, and the function
succeeds, the return value is zero, but the function does not clear the last
error information. This makes it difficult to determine success or failure. To
deal with this, you should clear the last error information by calling SetLastError(0) before calling SetWindowLong.
Then, function failure will be indicated by a return value of zero and a GetLastError
result that is nonzero.
Remarks
The SetWindowLong
function fails if the window specified by the hWnd parameter does not
belong to the same process as the calling thread.
If you use
the SetWindowLong function and the GWL_WNDPROC index to replace the
window procedure, the window procedure must conform to the guidelines specified
in the description of the WindowProc callback function.
Calling SetWindowLong
with the GWL_WNDPROC index creates a subclass of the window class used to
create the window. An application should not subclass a window created by
another process. The SetWindowLong function creates the window subclass
by changing the window procedure associated with a particular window, causing
Windows to call the new window procedure instead of the previous one. An
application must pass any messages not processed by the new window procedure to
the previous window procedure by calling CallWindowProc. This allows the
application to create a chain of window procedures.
Reserve extra
window memory by specifying a nonzero value in the cbWndExtra member of
the WNDCLASS
structure used with the RegisterClass function.
You must not
call SetWindowLong with the GWL_HWNDPARENT index to change the parent of
a child window. Instead, use the SetParent function.
See Also