WM_LBUTTONDOWN message
Posted when the user presses the left mouse button while the cursor is in the client area of a window. If the mouse is not captured, the message is posted to the window beneath the cursor. Otherwise, the message is posted to the window that has captured the mouse.
A window receives this message through its WindowProc function.
#define WM_LBUTTONDOWN 0x0201
Parameters
-
wParam
-
Indicates whether various virtual keys are down. This parameter can be one or more of the following values.
Value Meaning - MK_CONTROL
- 0x0008
The CTRL key is down. - MK_LBUTTON
- 0x0001
The left mouse button is down. - MK_MBUTTON
- 0x0010
The middle mouse button is down. - MK_RBUTTON
- 0x0002
The right mouse button is down. - MK_SHIFT
- 0x0004
The SHIFT key is down. - MK_XBUTTON1
- 0x0020
The first X button is down. - MK_XBUTTON2
- 0x0040
The second X button is down. -
lParam
-
The low-order word specifies the x-coordinate of the cursor. The coordinate is relative to the upper-left corner of the client area.
The high-order word specifies the y-coordinate of the cursor. The coordinate is relative to the upper-left corner of the client area.
Return value
If an application processes this message, it should return zero.
Example
LRESULT CALLBACK WndProc(_In_ HWND hWnd, _In_ UINT msg, _In_ WPARAM wParam, _In_ LPARAM lParam)
{
POINT pt;
switch (msg)
{
case WM_LBUTTONDOWN:
{
pt.x = GET_X_LPARAM(lParam);
pt.y = GET_Y_LPARAM(lParam);
}
break;
default:
return DefWindowProc(hWnd, msg, wParam, lParam);
}
return 0;
}
For more examples see Windows Classic Samples on GitHub.
Remarks
As noted above, the x-coordinate is in the low-order short of the return value; the y-coordinate is in the high-order short (both represent signed values because they can take negative values on systems with multiple monitors). If the return value is assigned to a variable, you can use the MAKEPOINTS macro to obtain a POINTS structure from the return value. You can also use the GET_X_LPARAM or GET_Y_LPARAM macro to extract the x- or y-coordinate.
Important
Do not use the LOWORD or HIWORD macros to extract the x- and y- coordinates of the cursor position because these macros return incorrect results on systems with multiple monitors. Systems with multiple monitors can have negative x- and y- coordinates, and LOWORD and HIWORD treat the coordinates as unsigned quantities.
To detect that the ALT key was pressed, check whether GetKeyState with VK_MENU < 0. Note, this must not be GetAsyncKeyState.
Requirements
| Requirement | Value |
|---|---|
| Minimum supported client |
Windows 2000 Professional [desktop apps only] |
| Minimum supported server |
Windows 2000 Server [desktop apps only] |
| Header |
|
See also
-
Reference
-
Conceptual
-
Other Resources