windows_programming_notes.nbk: Home | Index | Next Page: SetWindowLongPtr | Previous Page: SetTimer
#win32 functions
The SetWindowLong function changes an attribute of the specified window. The function also sets the 32-bit (long) value at the specified offset into the extra window memory.
Note This function has been superseded by the SetWindowLongPtr function. To write code that is compatible with both 32-bit and 64-bit versions of Microsoft Windows, use the SetWindowLongPtr function.
LONG SetWindowLong(
HWND hWnd,
int nIndex,
LONG dwNewLong
);
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.
Certain window data is cached, so changes you make using SetWindowLong will not take effect until you call the SetWindowPos function. Specifically, if you change any of the frame styles, you must call SetWindowPos with the SWP_FRAMECHANGED flag for the cache to be updated properly.
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 afterward. 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.
If the window has a class style of CS_CLASSDC or CS_OWNDC, do not set the extended window styles WS_EX_COMPOSITED or WS_EX_LAYERED.
Windows XP/Vista: Calling SetWindowLong to set the style on a progressbar will reset its position.
Windows 95/98/Me: SetWindowLongW is supported by the Microsoft Layer for Unicode (MSLU). SetWindowLongA is also supported to provide more consistent behavior across all Windows operating systems. To use these versions, you must add certain files to your application, as outlined in Microsoft Layer for Unicode on Windows 95/98/Me Systems .
For an example, see Subclassing a Window.
See also A list of window value offsets
windows_programming_notes.nbk: Home | Index | Next Page: SetWindowLongPtr | Previous Page: SetTimer
Notebook exported on Monday, 7 July 2008, 18:56:50 PM Eastern Daylight Time