C#中利用GDI作图解决异或问题

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Runtime.InteropServices;

namespace RubberBands
{
 /// <summary>
 /// Summary description for Form1.
 /// </summary>
 public class RubberBandForm : System.Windows.Forms.Form
 {

  /// API Declarations
  private struct POINTAPI {
   public int x;
   public int y;
  }

  [DllImport("gdi32.DLL")]
  private static extern Int32 SetROP2(IntPtr hDC,
   int nDrawMode);

  [DllImport("gdi32.dll")]
  private static extern bool MoveToEx(IntPtr hDC,
   int x,
   int y,
   POINTAPI lpPoint);

  [DllImport("gdi32.dll")]
  private static extern bool LineTo(IntPtr hDC,
   int x,
   int y);

  [DllImport("gdi32.dll")]
  private static extern IntPtr CreatePen(int nPenStyle,
   int nWidth,
   int crColor);

  [DllImport("gdi32.dll")]
  private static extern IntPtr SelectObject(IntPtr hDC,
   IntPtr hObject);

  [DllImport("gdi32.dll")]
  private static extern bool DeleteObject(IntPtr hObject);

  private const int R2_NOT = 6;
  private const int R2_XORPEN = 7;

  private const int PS_DOT = 2;

  /// <summary>
  /// Required designer variable.
  /// </summary>
  private System.ComponentModel.Container components = null;
  private System.Drawing.Graphics m_oDrawingSurface;
  private POINTAPI m_ptsStart = new POINTAPI();
  private POINTAPI m_ptsPrevious;

  public RubberBandForm()
  {
   //
   // Required for Windows Form Designer support
   //
   InitializeComponent();

   //Add any initialization after the InitializeComponent() call
   m_oDrawingSurface = this.CreateGraphics();

  }

  /// <summary>
  /// Clean up any resources being used.
  /// </summary>
  protected override void Dispose( bool disposing )
  {
   if( disposing )
   {
    if (components != null)
    {
     components.Dispose();
    }
   }
   base.Dispose( disposing );
  }

  protected override void OnMouseDown(MouseEventArgs e) {
   // Check whether the left mouse button
   // has generated the associated event.
   if (e.Button == MouseButtons.Left) {
    // Store the starting point for your rubber-band line.
    m_ptsStart.x = e.X;
    m_ptsStart.y = e.Y;
    // Store the previous end point for your rubber-band line.
    m_ptsPrevious = m_ptsStart;
   }

   base.OnMouseDown (e);
  }

  protected override void OnMouseMove(MouseEventArgs e) {
   // Check if the left mouse button is pressed.
   
   if( e.Button == MouseButtons.Left) {
    // Declare local variables.
    // handle to an appropriate device context obtained
    // from m_oDrawingSurface
    IntPtr hDC;
    // previous mix mode; returned from SetROP2
    Int32 Mix ;
    // temporary variables used for rubber-banding logic
    POINTAPI ptsCurrent;
    POINTAPI ptsOld = new POINTAPI();
    // determines whether MoveToEx and LineTo calls are successful
    Boolean Success;

    // Store the current end point of your rubber-band line.
    ptsCurrent.x = e.X;
    ptsCurrent.y = e.Y;

    // Perform the requisite graphical operations.
    // Obtain a handle to an appropriate device context
    hDC = m_oDrawingSurface.GetHdc();
    // Set the raster mode to invert the screen color
    Mix = SetROP2(hDC, R2_NOT);
    // Move to the starting point of the rubber-band line
    Success = MoveToEx(hDC, m_ptsStart.x, m_ptsStart.y, ptsOld);
    // Erase the previous line
    Success = LineTo(hDC, m_ptsPrevious.x, m_ptsPrevious.y);
    // Move to the starting point of the rubber-band line
    Success = MoveToEx(hDC, m_ptsStart.x, m_ptsStart.y, ptsOld);
    // Draw the new line
    Success = LineTo(hDC, ptsCurrent.x, ptsCurrent.y);
    // Release the obtained handle to a device context
    m_oDrawingSurface.ReleaseHdc(hDC);

    // Store the current end point for the next erase operation
    m_ptsPrevious = ptsCurrent;
    }

   base.OnMouseMove (e);
  }

  #region Windows Form Designer generated code
  /// <summary>
  /// Required method for Designer support - do not modify
  /// the contents of this method with the code editor.
  /// </summary>
  private void InitializeComponent()
  {
   //
   // RubberBandForm
   //
   this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
   this.BackColor = System.Drawing.SystemColors.Window;
   this.ClientSize = new System.Drawing.Size(350, 286);
   this.Name = "RubberBandForm";
   this.Text = "Using Unmanaged APIs";

  }
  #endregion

  /// <summary>
  /// The main entry point for the application.
  /// </summary>
  [STAThread]
  static void Main()
  {
   Application.Run(new RubberBandForm());
  }
 }
}