g.Clear(Color.White);

------解决方案--------------------
桌面么？
那你一定得到桌面的句柄了。
C# code
[DllImport("user32.dll", EntryPoint="InvalidateRect")]
public static extern int InvalidateRect (
    IntPtr hwnd,
    Rectangle lpRect,
    bool bErase
);
[DllImport("user32.dll", EntryPoint="GetDesktopWindow")]
public static extern IntPtr GetDesktopWindow ();

//调用
InvalidateRect(GetDesktopWindow(),System.Windows.Forms.Screen.PrimaryScreen.Bounds,true);

------解决方案--------------------
重绘桌面：
C# code
[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
public static extern IntPtr GetDesktopWindow();

[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
public static extern bool RedrawWindow(IntPtr hwnd, RECT rcUpdate, IntPtr hrgnUpdate, int flags);

RedrawWindow(GetDesktopWindow(), null, IntPtr.Zero, 0x85);



------解决方案--------------------

新建控制台程序。粘贴：
C# code

[DllImport("user32.dll", EntryPoint = "InvalidateRect")]
public static extern int InvalidateRect(
    IntPtr hwnd,
    IntPtr lpRect,
    bool bErase
);
[DllImport("user32.dll", EntryPoint = "GetDesktopWindow")]
public static extern IntPtr GetDesktopWindow();

[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
public static extern bool RedrawWindow(IntPtr hwnd, ref Rectangle rcUpdate, IntPtr hrgnUpdate, int flags);



static void Main(string[] args)
{
    Thread.Sleep(2000);
    Console.WriteLine("现在开始绘制屏幕");
    Graphics g = Graphics.FromHwnd(IntPtr.Zero);
    g.DrawLine(Pens.Yellow, new Point(0, 0), new Point(500, 800));
    g.Dispose(); 
    Thread.Sleep(2000);
    Console.WriteLine("现在开始清除屏幕");
    Rectangle rect = Screen.PrimaryScreen.Bounds;
    RedrawWindow(GetDesktopWindow(), ref rect, IntPtr.Zero, 0x85);
    Console.ReadKey();
}