各位大虾，请问在.Net如何实现在屏幕上任何地方画图？

[sbsummer]

我利用了USER32.DLL和GDI32.DLL,为什么这样看不到矩形？？
能否给出提示，谢谢
IntPtr hDC = PlatformInvokeUSER32.GetDC(PlatformInvokeUSER32.GetDesktopWindow());
Graphics m_Graphics=Graphics.FromHdc(hDC);
Pen redPen=new Pen(Color.Red, 10);
Rectangle rWorkArea = Screen.GetWorkingArea(Screen.PrimaryScreen.WorkingArea);
m_Graphics.DrawRectangle(redPen,rWorkArea);
PlatformInvokeUSER32.ReleaseDC(PlatformInvokeUSER32.GetDesktopWindow(), hDC);

[sbsummer]

[DllImport("User32.dll")]

public extern static System.IntPtr GetDC(System.IntPtr hWnd);



private void button1_Click(object sender, System.EventArgs e)

{

System.IntPtr DesktopHandle = GetDC(System.IntPtr.Zero);

Graphics g = System.Drawing.Graphics.FromHdc(DesktopHandle);

g.FillRectangle(new SolidBrush(Color.Red),0,0,100,100);

}