Need an expert advice regarding GDI & C#

[vitagoni]

Ok, here is the example.

Why XORCycleA doesn't work while XORCycleB works...

Here is quite serious riddle for an expert.

Hope you can "speak" C# and will understand my strings without additional comments )

XOR_Paint will be invoked each time you move the dialog window and smth overlays it.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;
using System.Drawing;
using System.Windows.Forms;
using System.Drawing.Imaging;

namespace ConsoleTestApplication
{
 class Program
 {
 static void Main(string[] args)
 {
  new XOR().ShowDialog();
 }
 }

 internal class XOR : Form
 {
 [DllImport("gdi32.dll")]
 public static extern bool BitBlt(IntPtr hdcDest, int nXDest, int nYDest, int nWidth, int nHeight, IntPtr hdcSrc,
            int nXSrc, int nYSrc, Int32 dwRop);
 [DllImport("gdi32.dll")]
 public static extern bool TextOut(IntPtr hdc, int x, int y, string lpString, int cbString);

 const Int32 SRCCOPY = 0x00CC0020;
 const Int32 SRCAND = 0x008800C6;
 const Int32 SRCINVERT = 0x00660046; // INVERT means XOR

 public XOR()
 {
  // Bit map created once with screen data in it
  bmA = new Bitmap(s.Width, s.Height, PixelFormat.Format32bppRgb);
  Graphics gfxBMP = Graphics.FromImage(bmA);
  gfxBMP.CopyFromScreen(0, 0, 0, 0, new Size(s.Width, s.Height), CopyPixelOperation.SourceCopy);
  gfxBMP.Dispose();
  this.Paint += new PaintEventHandler(XOR_Paint);
 }

 void XOR_Paint(object sender, PaintEventArgs e)
 {
  xorCycleA();
  xorCycleB();
 }

 Bitmap bmA;
 Size s = Screen.PrimaryScreen.Bounds.Size;
 unsafe void xorCycleA()
 {
  Graphics grfxScreen = Graphics.FromHwnd(IntPtr.Zero);
  Graphics grfxBitmap = Graphics.FromImage(bmA);
  bmA.Save(@"c:\before_xorA.bmp", ImageFormat.Bmp);
  IntPtr hdcBitmap = grfxBitmap.GetHdc();
  IntPtr hdcScreen = grfxScreen.GetHdc();
  // without any changes just copy screen data with XOR
  BitBlt(hdcBitmap, 0, 0, bmA.Width, bmA.Height, hdcScreen, 0, 0, SRCINVERT);
  grfxScreen.ReleaseHdc(hdcScreen);
  grfxBitmap.ReleaseHdc(hdcBitmap);
  grfxScreen.Dispose();
  grfxBitmap.Dispose();
  bmA.Save(@"c:\after_xorA.bmp", ImageFormat.Bmp);
 }

 Bitmap bmB;
 unsafe void xorCycleB()
 {
  Graphics grfxScreen = Graphics.FromHwnd(IntPtr.Zero);
  bmB = new Bitmap((int)grfxScreen.VisibleClipBounds.Width,
    (int)grfxScreen.VisibleClipBounds.Height, grfxScreen);
  Graphics grfxBitmap = Graphics.FromImage(bmB);
  bmB.Save(@"c:\before_xorB.bmp", ImageFormat.Bmp);
  IntPtr hdcBitmap = grfxBitmap.GetHdc();
  IntPtr hdcScreen = grfxScreen.GetHdc();
  BitBlt(hdcBitmap, 0, 0, bmB.Width, bmB.Height, hdcScreen, 0, 0, SRCCOPY);
  TextOut(hdcBitmap, 100, 100, "Hi, I'm here", 12);
  // comment BitBlt to make sure the bitmap contains screen data and the string on it inside after_xorB.bmp
  BitBlt(hdcBitmap, 0, 0, bmB.Width, bmB.Height, hdcScreen, 0, 0, SRCINVERT);
  grfxScreen.ReleaseHdc(hdcScreen);
  grfxBitmap.ReleaseHdc(hdcBitmap);
  grfxScreen.Dispose();
  grfxBitmap.Dispose();
  bmB.Save(@"c:\after_xorB.bmp", ImageFormat.Bmp);
 }
 }
}