I draw lines on the desktop and want to remove the old ones to make a rotation look in lines?

[DllImport("User32.dll")]
public static extern IntPtr GetDC(IntPtr hwnd);
[DllImport("User32.dll")]
public static extern void ReleaseDC(IntPtr hwnd, IntPtr dc);
//Get a Graphics object for the entire screen and draw a rectangle with it:

int currentLine = 0, totalNumOfLines=12;

Point point = new Point(60, 60);

private void FormSettings_Load(object sender, EventArgs e)
{
    desktopPtr = GetDC(IntPtr.Zero);
    g = Graphics.FromHdc(desktopPtr);
}

IntPtr desktopPtr;

Graphics g;

private void FormSettings_FormClosed(object sender, FormClosedEventArgs e)
{
    g.Dispose();
    ReleaseDC(IntPtr.Zero, desktopPtr);
}

private void timerDraw_Tick(object sender, EventArgs e)
{
    g.Dispose();
    ReleaseDC(IntPtr.Zero, desktopPtr);

    desktopPtr = GetDC(IntPtr.Zero);
    g = Graphics.FromHdc(desktopPtr);

    currentLine++;

    if (currentLine == totalNumOfLines) currentLine = 0;

    g.DrawLine(new Pen(Color.Yellow, 2), point.X, point.Y
        , point.X + (int)(50 * Math.Cos((currentLine / (double)totalNumOfLines) * Math.PI * 2))
        , point.Y + (int)(50 * Math.Sin((currentLine / (double)totalNumOfLines) * Math.PI * 2)));
}