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WM_PAINT message

The WM_PAINT message is sent when the system or another application makes a request to paint a portion of an application's window. The message is sent when the UpdateWindow or RedrawWindow function is called, or by the DispatchMessage function when the application obtains a WM_PAINT message by using the GetMessage or PeekMessage function.

A window receives this message through its WindowProc function.


LRESULT CALLBACK WindowProc(
  HWND hwnd, 
  UINT  uMsg, 
  WPARAM wParam, 
  LPARAM lParam     
);

Parameters

wParam

This parameter is not used.

lParam

This parameter is not used.

Return value

An application returns zero if it processes this message.

Remarks

The WM_PAINT message is generated by the system and should not be sent by an application. To force a window to draw into a specific device context, use the WM_PRINT or WM_PRINTCLIENT message. Note that this requires the target window to support the WM_PRINTCLIENT message. Most common controls support the WM_PRINTCLIENT message.

The DefWindowProc function validates the update region. The function may also send the WM_NCPAINT message to the window procedure if the window frame must be painted and send the WM_ERASEBKGND message if the window background must be erased.

The system sends this message when there are no other messages in the application's message queue. DispatchMessage determines where to send the message; GetMessage determines which message to dispatch. GetMessage returns the WM_PAINT message when there are no other messages in the application's message queue, and DispatchMessage sends the message to the appropriate window procedure.

A window may receive internal paint messages as a result of calling RedrawWindow with the RDW_INTERNALPAINT flag set. In this case, the window may not have an update region. An application should call the GetUpdateRect function to determine whether the window has an update region. If GetUpdateRect returns zero, the application should not call the BeginPaint and EndPaint functions.

An application must check for any necessary internal painting by looking at its internal data structures for each WM_PAINT message, because a WM_PAINT message may have been caused by both a non-NULL update region and a call to RedrawWindow with the RDW_INTERNALPAINT flag set.

The system sends an internal WM_PAINT message only once. After an internal WM_PAINT message is returned from GetMessage or PeekMessage or is sent to a window by UpdateWindow, the system does not post or send further WM_PAINT messages until the window is invalidated or until RedrawWindow is called again with the RDW_INTERNALPAINT flag set.

For some common controls, the default WM_PAINT message processing checks the wParam parameter. If wParam is non-NULL, the control assumes that the value is an HDC and paints using that device context.

Requirements

Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]

Header

Winuser.h (include Windows.h)

See also

Painting and Drawing Overview
Painting and Drawing Messages
BeginPaint
DefWindowProc
DispatchMessage
EndPaint
GetMessage
GetUpdateRect
PeekMessage
RedrawWindow
UpdateWindow
WM_ERASEBKGND
WM_NCPAINT
WM_PRINT
WM_PRINTCLIENT

 

 

Build date: 6/12/2013

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