C#中利用GDI作图解决异或问题

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

namespace RubberBands
{
 ///

  /// 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;

  ///

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

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

  }

  ///

  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);
  }

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