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C++

I wrote a similar example of sliding control, the results of each slide the slider, there appeared the following chart, from Figure 1, after sliding to the left block in Figure 2.
Figure I

Figure 2

Chart below is the main code block:

void SliderBoth::SetBlockColor()
CClientDC dc(this);
CRect rc;
GetClientRect(&rc);
int iNumTic=(pos.iMax-pos.iMin)/pos.iTick;
const int iL=MARGIN_SLIDE;
const int iR=rc.right-MARGIN_SLIDE;
const double dbStep=(iR-iL)*1.0/iNumTic;
double db=iL;
CBrush brush;   
brush.CreateSolidBrush(RGB(165,196,211));   
CBrush * pOldBrush= dc.SelectObject (&brush);

dc.Rectangle((int)lval+MARGIN_SLIDE*2,rc.top+1,(int)rval,rc.bottom-MARGIN_SLIDE*2);  
dc.SelectObject (pOldBrush);
brush.DeleteObject();
#endif
}

How can we solve this problem, obtain the DC I have problems?
Please help, thank you!