SetWindowRgn function (winuser.h)

Article
02/22/2024

The SetWindowRgn function sets the window region of a window. The window region determines the area within the window where the system permits drawing. The system does not display any portion of a window that lies outside of the window region

Syntax

int SetWindowRgn(
  [in] HWND hWnd,
  [in] HRGN hRgn,
  [in] BOOL bRedraw
);

Parameters

[in] hWnd

A handle to the window whose window region is to be set.

[in] hRgn

A handle to a region. The function sets the window region of the window to this region.

If hRgn is NULL, the function sets the window region to NULL.

[in] bRedraw

Specifies whether the system redraws the window after setting the window region. If bRedraw is TRUE, the system does so; otherwise, it does not.

Typically, you set bRedraw to TRUE if the window is visible.

Return value

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero.

Remarks

When this function is called, the system sends the WM_WINDOWPOSCHANGING and WM_WINDOWPOSCHANGED messages to the window.

The coordinates of a window's window region are relative to the upper-left corner of the window, not the client area of the window.

Note If the window layout is right-to-left (RTL), the coordinates are relative to the upper-right corner of the window. See Window Layout and Mirroring.

After a successful call to SetWindowRgn, the system owns the region specified by the region handle hRgn. The system does not make a copy of the region. Thus, you should not make any further function calls with this region handle. In particular, do not delete this region handle. The system deletes the region handle when it no longer needed.

To obtain the window region of a window, call the GetWindowRgn function.

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	winuser.h (include Windows.h)
Library	User32.lib
DLL	User32.dll
API set	ext-ms-win-ntuser-draw-l1-1-0 (introduced in Windows 8)