struct LINE
{
int xp[2];
int yp[2];
};
LINE L[100];

int Lnum = 0;
int icoord = 0;
int isline =0;
int psize = 1;


if(bLineFlag)
{
CPen penRed(PS_SOLID,1,RGB(250,0,0));
CPen *pOldPen = NULL;
pOldPen = pDC->SelectObject(&penRed);
CRect rcClient;
GetClientRect(&rcClient);
CPoint xpoint,ypoint;

L[Lnum].xp[icoord] = point.x / psize;
L[Lnum].yp[icoord] = point.y / psize;
if(icoord ==0)
{
icoord++; 
isline = 0;
}
else
{
isline =1;
pDC->MoveTo(L[Lnum].xp[0],L[Lnum].yp[0]);
t.x = L[Lnum].xp[0];
t.y = L[Lnum].yp[0];

pDC->LineTo(L[Lnum].xp[1],L[Lnum].yp[1]);
p.x = L[Lnum].xp[1];
p.y = L[Lnum].yp[1];

icoord =0;
}
DrawCross(point,RGB(0,255,0),5,pDC->m_hDC);
}

///On button click
void CMyWindow::DrawLineOnClick()
{
	CClientDC dc(this);

             //a function called to draw a line
	DrawLine(dc);
//         Create this variable in public and initialize it to false in constructor
	bClicked=TRUE;
	Invalidate(TRUE);

}

//this function draws a line when button is clicked
void CMyWindow::DrawLine(CDC& dc)
{
	CRect rect;
	GetClientRect(&rect);
	dc.MoveTo(0,0);
	dc.LineTo(rect.Width(),rect.Height());
	
}

//this piece of code will satisfy your need... if u r working in SDI u write it in OnDraw() function else if u use Dlg based write in OnPaint()

void CMyWindow::OnPaint()
{
	CPaintDC dc(this);
	if(TRUE==bClicked)
	{
	DrawLine(dc);
	}
}

//sure it works...