The WM_MOUSEWHEEL message is sent to the focus window when the mouse wheel is rotated. The DefWindowProc function propagates the message to the window's parent. There should be no internal forwarding of the message, since DefWindowProc propagates it up the parent chain until it finds a window that processes it.

A window receives this message through its WindowProc function.

Syntax

WM_MOUSEWHEEL

    WPARAM wParam
    LPARAM lParam;
    

Parameters

wParam

The high-order word indicates the distance the wheel is rotated, expressed in multiples or divisions of WHEEL_DELTA, which is 120. A positive value indicates that the wheel was rotated forward, away from the user; a negative value indicates that the wheel was rotated backward, toward the user.

The low-order word indicates whether various virtual keys are down. This parameter can be one or more of the following values.

MK_CONTROL

The CTRL key is down.

MK_LBUTTON

The left mouse button is down.

MK_MBUTTON

The middle mouse button is down.

MK_RBUTTON

The right mouse button is down.

MK_SHIFT

The SHIFT key is down.

MK_XBUTTON1

Windows 2000/XP: The first X button is down.

MK_XBUTTON2

Windows 2000/XP: The second X button is down.

lParam

The low-order word specifies the x-coordinate of the pointer, relative to the upper-left corner of the screen.

The high-order word specifies the y-coordinate of the pointer, relative to the upper-left corner of the screen.

Return Value

If an application processes this message, it should return zero.

Remarks

Use the following code to get the information in the wParam parameter:

fwKeys = GET_KEYSTATE_WPARAM(wParam);
zDelta = GET_WHEEL_DELTA_WPARAM(wParam);

Use the following code to obtain the horizontal and vertical position:

xPos = GET_X_LPARAM(lParam); 
yPos = GET_Y_LPARAM(lParam); 

You can also use the MAKEPOINTS macro to convert the lParam parameter to a POINTS structure.

The wheel rotation will be a multiple of WHEEL_DELTA, which is set at 120. This is the threshold for action to be taken, and one such action (for example, scrolling one increment) should occur for each delta.

The delta was set to 120 to allow Microsoft or other vendors to build finer-resolution wheels in the future, including perhaps a freely-rotating wheel with no notches. The expectation is that such a device would send more messages per rotation, but with a smaller value in each message. To support this possibility, you should either add the incoming delta values until WHEEL_DELTA is reached (so for a delta-rotation you get the same response), or scroll partial lines in response to the more frequent messages. You could also choose your scroll granularity and accumulate deltas until it is reached.

Windows 95, Windows NT 3.51: Support for the mouse wheel is provided through a separately-running module, MSWheel, that generates a MSH_MOUSEWHEEL message. The MSWheel module, which consists of MSWheel.exe and MSWheel.dll, is installed with the IntelliPoint software that is shipped with the IntelliMouse pointing device. In addition, MSH_MOUSEWHEEL is defined in the header file (ZMouse.h) that an application must use to implement support for the wheel via the MSWheel module.

MSH_MOUSEWHEEL
zDelta = (int) wParam; // wheel rotation 
xPos = LOWORD(lParam); // horizontal position of pointer 
yPos = HIWORD(lParam); // vertical position of pointer 

Note, there is no fwKeys for MSH_MOUSEWHEEL. Otherwise, the parameters are exactly the same as for WM_MOUSEWHEEL.

It is up to the application to forward MSH_MOUSEWHEEL to any embedded objects or controls. The application is required to send the message to an active embedded OLE application. It is optional that the application sends it to a wheel-enabled control with focus. If the application does send the message to a control, it can check the return value to see if the message was processed. Controls are required to return a value of TRUE if they process the message.

Notification Requirements

Minimum DLL Version	None
Header	Declared in Winuser.h, include Windows.h
Minimum operating systems	Windows 98, Windows NT 4.0

See Also

Mouse Input Overview, GET_KEYSTATE_WPARAM, GET_X_LPARAM, GET_Y_LPARAM, GetSystemMetrics, GET_WHEEL_DELTA_WPARAM, HIWORD, LOWORD, MAKEPOINTS, mouse_event, POINTS, SystemParametersInfo