SetWindowSubclass function (commctrl.h)

Installs or updates a window subclass callback.


BOOL SetWindowSubclass(
  [in] HWND         hWnd,
  [in] SUBCLASSPROC pfnSubclass,
  [in] UINT_PTR     uIdSubclass,
  [in] DWORD_PTR    dwRefData


[in] hWnd

Type: HWND

The handle of the window being subclassed.

[in] pfnSubclass


A pointer to a window procedure. This pointer and the subclass ID uniquely identify this subclass callback. For the callback function prototype, see SUBCLASSPROC.

[in] uIdSubclass


The subclass ID. This ID together with the subclass procedure uniquely identify a subclass. To remove a subclass, pass the subclass procedure and this value to the RemoveWindowSubclass function. This value is passed to the subclass procedure in the uIdSubclass parameter.

[in] dwRefData


DWORD_PTR to reference data. The meaning of this value is determined by the calling application. This value is passed to the subclass procedure in the dwRefData parameter. A different dwRefData is associated with each combination of window handle, subclass procedure and uIdSubclass.

Return value

Type: BOOL

TRUE if the subclass callback was successfully installed; otherwise, FALSE.


Subclass callbacks are identified by the combination of the callback address and the caller-defined subclass ID. If the callback address and ID pair have not yet been installed, then this function installs the subclass. If the pair has already been installed, then this function just updates the reference data.

Each callback can store a single DWORD_PTR of reference data, which is passed to the callback function when it is called to filter messages. No reference counting is performed for the callback; it may repeatedly call SetWindowSubclass to alter the value of its reference data element.

Warning  You cannot use the subclassing helper functions to subclass a window across threads.


Requirement Value
Minimum supported client Windows XP [desktop apps only]
Minimum supported server Windows Server 2003 [desktop apps only]
Target Platform Windows
Header commctrl.h
Library Comctl32.lib
DLL Comctl32.dll (version 5.8 or later)
API set ext-ms-win-shell-comctl32-window-l1-1-0 (introduced in Windows 10, version 10.0.14393)

See also