Paul Welter's Weblog

using System;
using System.Runtime.InteropServices;
using System.Text;
using System.Windows.Forms;

namespace TextUndoBuffer
{
 /// <summary>
 /// Work around for KB 812943, The RichEdit Control Undo 
 /// Information May Be Lost When the Control Retrieves Text.
 /// </summary>
 public class RichTextBoxEx : RichTextBox
 {
     [DllImport("user32.dll", EntryPoint="SendMessage", CharSet=CharSet.Auto )]
     static extern int SendMessage(IntPtr hWnd, int msg, 
             ref GETTEXTEX wParam, System.Text.StringBuilder lParam);
     [DllImport("user32.dll", EntryPoint="SendMessage", CharSet=CharSet.Auto )]
     static extern int SendMessage(IntPtr hWnd, int msg, 
             ref GETTEXTLENGTHEX wParam, int lParam);

     [StructLayout(LayoutKind.Sequential)]
     struct GETTEXTEX
     {
         public Int32    cb;
         public Int32    flags;
         public Int32    codepage;
         public IntPtr   lpDefaultChar;
         public IntPtr   lpUsedDefChar;
     }

     [StructLayout(LayoutKind.Sequential)]
     struct GETTEXTLENGTHEX
     {
         public Int32    flags;
         public Int32    codepage;
     }
     const int WM_USER = 0x0400;

     // RichEdit messages (Richedit.h)
     const int EM_GETTEXTEX =(WM_USER + 94);
     const int EM_GETTEXTLENGTHEX =(WM_USER + 95);
     // Flags for the GETEXTEX data structure  
     const int GT_DEFAULT = 0; 
     // Flags for the GETTEXTLENGTHEX data structure 
     const int GTL_DEFAULT = 0; // Do default (return # of chars) 
     const int GTL_CLOSE = 4; // Fast computation of a "close" answer 

     public RichTextBoxEx()
     {
     }

     public override string Text
     {
         get
         {
             GETTEXTLENGTHEX getLength = new GETTEXTLENGTHEX();
             getLength.flags = GTL_CLOSE; //get buffer size
             getLength.codepage = 1200; //Unicode

             int textLength = SendMessage(base.Handle, 
                    EM_GETTEXTLENGTHEX, 
                    ref getLength, 0);
         
             GETTEXTEX getText = new GETTEXTEX();
             getText.cb = textLength+2; //add space for null terminator
             getText.flags = GT_DEFAULT;
             getText.codepage = 1200; //Unicode

             StringBuilder sb = new StringBuilder(getText.cb);

             SendMessage(base.Handle, 
                    EM_GETTEXTEX, 
                    ref getText, sb);
         
              return sb.ToString();
         }
         set
         {
             base.Text = value;
         }
     }

     public override int TextLength
     {
         get
         {
             GETTEXTLENGTHEX getLength = new GETTEXTLENGTHEX();
             getLength.flags = GTL_DEFAULT; //Returns the number of characters
             getLength.codepage = 1200; //Unicode

             return SendMessage(base.Handle, 
                    EM_GETTEXTLENGTHEX, 
                    ref getLength, 0);                
         }
     }
 }
}