The MAKELONG macro creates an unsigned 32-bit value by concatenating two given 16-bit values.

DWORD MAKELONG(
WORD wLow, // low-order word of long value
WORD wHigh // high-order word of long value
);

Parameters
wLow
Specifies the low-order word of the new long value.
wHigh
Specifies the high-order word of the new long value.
Return Values
The return value is an unsigned 32-bit value.

Remarks
The MAKELONG macro is defined as follows:

#define MAKELONG(a, b) \
((LONG) (((WORD) (a)) | ((DWORD) ((WORD) (b))) << 16))

DWORD MAKELONG(
WORD wLow, // low-order word of long value
WORD wHigh // high-order word of long value
);

SendMessage(hWnd,WM_LBUTTONDOWN,NULL,MAKELONG(x*cxBlock,y*cyBlock));

处理消息： x = LOWORD(lParam)/cxBlock;
y = HIWORD(lParam)/cyBlock;