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
);
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. |
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.
The SetWindowLong function fails if the window specified by the hWnd parameter does not belong to the same process as the calling thread.
Certain window data is cached, so changes you make using SetWindowLong will not take effect until you call the SetWindowPos function.
If you use SetWindowLong with 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.
If you use SetWindowLong with the DWL_MSGRESULT index to set the return value for a message processed by a dialog procedure, you should return TRUE directly afterwards. Otherwise, if you call any function that results in your dialog procedure receiving a window message, the nested window message could overwrite the return value you set using DWL_MSGRESULT.
Calling SetWindowLong with the GWL_WNDPROC index creates a subclass of the window class used to create the window. An application can subclass a system class, but should not subclass a window class created by another process. The SetWindowLong function creates the window subclass by changing the window procedure associated with a particular window class, causing the system 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 WNDCLASSEX structure used with the RegisterClassEx function.
You must not call SetWindowLong with the GWL_HWNDPARENT index to change the parent of a child window. Instead, use the SetParent function.
Windows CE: The nIndex parameter must be a multiple of 4 bytes.
Unaligned access is not supported.
The following nIndex parameter values are not supported:
GWL_HINSTANCE
GWL_HWNDPARENT
GWL_USERDATA
Windows CE versions 2.0 and later support the DWL_DLGPROC value in the nIndex parameter, but Windows CE 1.0 does not.
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Requires version 1.0 or later.
Header: Declared in winuser.h.
Import Library: Use user32.lib.
Unicode: Implemented as Unicode and ANSI versions on Windows NT.
Window Classes Overview, Window Class Functions, CallWindowProc, GetWindowLong, RegisterClassEx, SetParent, WindowProc, WNDCLASSEX