这是MSDN上的解释：DC_PEN：Windows 2000/XP: Solid pen color. The default color is white. The color can be changed by using the SetDCPenColor function. For more information, see the Remarks section. NULL_PEN：NULL pen. The null pen draws nothing. 看样子，DC_PEN的颜色可以随便改。而NULL_PEN，则画不出东西。
