//画线

CPen penBlack;
penBlack.CreatePen(PS_SOLID, 10, RGB(255, 0, 0));
CPen* pOldPen = pDC->SelectObject(&penBlack);

static int a, b;
srand((unsigned)time(NULL));

a = rand() % 200;
b = rand() % 200;
pDC->MoveTo(a,b);
pDC->LineTo(40, 40);


static int c = 1;//这里的图不会被擦出，
if (c == 1){
   pDC->MoveTo(300, 300);
   pDC->LineTo(340, 340);
   c++;

}

pDC->SelectObject(pOldPen);

RECT rect;
rect.left = 0;
rect.top = 0;
rect.right = 200;
rect.bottom = 200;

InvalidateRect(&rect, true);//使rect区域无效并擦出rect区域原来的图
//InvalidateRect(&rect, true);//使rect区域无效并擦出rect区域并引发WM_PAINT

当移动或改变窗口大小时，pDC->MoveTo(300, 300);pDC->LineTo(340, 340);这里画的线会被擦出。说明移动或改变窗口大小是使整个窗口无效并重绘。

InvalidateRect