C#在屏幕上画图

[DllImport("user32.dll")]

private static extern int GetDC(int hwnd);

private void button1_Click(object sender, EventArgs e)

{

System.IntPtr p = (IntPtr)GetDC(0);// '取得屏幕

Graphics g= Graphics.FromHdc(p);

g.DrawRectangle(new Pen(Color.Black),new Rectangle (100,100,100,100));

}

可能用到的API有：

[DllImport("user32.dll", EntryPoint = "GetDCEx", CharSet = CharSet.Auto, ExactSpelling = true)]

private static extern IntPtr IntGetDCEx(HandleRef hWnd, HandleRef hrgnClip, int flags);

[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]

public static extern IntPtr GetDesktopWindow();

[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]

private static extern int IntReleaseDC(HandleRef hWnd, HandleRef hDC);

[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]

private static extern bool RedrawWindow(HandleRef hwnd, ref RECT rcUpdate, HandleRef hrgnUpdate, int flags);

给你个在桌上画圆的代码吧:

[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]

public static extern IntPtr GetDesktopWindow();

[DllImport("user32.dll", EntryPoint = "GetDCEx", CharSet = CharSet.Auto, ExactSpelling = true)]

private static extern IntPtr GetDCEx(IntPtr hWnd, IntPtr hrgnClip, int flags);

private void button1_Click(object sender, EventArgs e)

{

IntPtr desk = GetDesktopWindow();

IntPtr deskDC = GetDCEx(desk, IntPtr.Zero, 0x403);

Graphics g = Graphics.FromHdc(deskDC);

g.FillEllipse(SystemBrushes.ControlText, 0, 0, 100, 100);

}

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);

[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);

}