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

wParam

Indicates whether various virtual keys are down. This parameter can be one or more of the following values.

Expand table

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 XBUTTON1 is down.
MK_XBUTTON2 0x0040	The XBUTTON2 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.

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

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.

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.

To detect that the ALT key was pressed, check whether GetKeyState with VK_MENU < 0. Note, this must not be GetAsyncKeyState.

Reference

GET_X_LPARAM

GET_Y_LPARAM

GetCapture

GetKeyState

SetCapture

WM_LBUTTONDBLCLK

WM_LBUTTONUP

Conceptual

Mouse Input

Other Resources

MAKEPOINTS

POINTS