Fix for the RichTextBox Undo Information Lost When Retrieving Text

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)]
 </span><span style="COLOR: #0000ff">struct</span><span style="COLOR: #000000"> GETTEXTEX
 {
     </span><span style="COLOR: #0000ff">public</span><span style="COLOR: #000000"> Int32    cb;
     </span><span style="COLOR: #0000ff">public</span><span style="COLOR: #000000"> Int32    flags;
     </span><span style="COLOR: #0000ff">public</span><span style="COLOR: #000000"> Int32    codepage;
     </span><span style="COLOR: #0000ff">public</span><span style="COLOR: #000000"> IntPtr   lpDefaultChar;
     </span><span style="COLOR: #0000ff">public</span><span style="COLOR: #000000"> IntPtr   lpUsedDefChar;
 }

 [StructLayout(LayoutKind.Sequential)]
 </span><span style="COLOR: #0000ff">struct</span><span style="COLOR: #000000"> GETTEXTLENGTHEX
 {
     </span><span style="COLOR: #0000ff">public</span><span style="COLOR: #000000"> Int32    flags;
     </span><span style="COLOR: #0000ff">public</span><span style="COLOR: #000000"> Int32    codepage;
 }
 </span><span style="COLOR: #0000ff">const</span><span style="COLOR: #000000"> </span><span style="COLOR: #0000ff">int</span><span style="COLOR: #000000"> WM_USER </span><span style="COLOR: #000000">=</span><span style="COLOR: #000000"> </span><span style="COLOR: #000000">0x0400</span><span style="COLOR: #000000">;

 </span><span style="COLOR: #008000">//</span><span style="COLOR: #008000"> RichEdit messages (Richedit.h)</span><span style="COLOR: #008000">

     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()
{
}
 </span><span style="COLOR: #0000ff">public</span><span style="COLOR: #000000"> </span><span style="COLOR: #0000ff">override</span><span style="COLOR: #000000"> </span><span style="COLOR: #0000ff">string</span><span style="COLOR: #000000"> Text
 {
     </span><span style="COLOR: #0000ff">get</span><span style="COLOR: #000000">
     {
         GETTEXTLENGTHEX getLength </span><span style="COLOR: #000000">=</span><span style="COLOR: #000000"> </span><span style="COLOR: #0000ff">new</span><span style="COLOR: #000000"> GETTEXTLENGTHEX();
         getLength.flags </span><span style="COLOR: #000000">=</span><span style="COLOR: #000000"> GTL_CLOSE; </span><span style="COLOR: #008000">//</span><span style="COLOR: #008000">get buffer size</span><span style="COLOR: #008000">

             getLength.codepage = 1200; //Unicode

int textLength = SendMessage(base.Handle,
EM_GETTEXTLENGTHEX,
ref getLength, 0);
         GETTEXTEX getText </span><span style="COLOR: #000000">=</span><span style="COLOR: #000000"> </span><span style="COLOR: #0000ff">new</span><span style="COLOR: #000000"> GETTEXTEX();
         getText.cb </span><span style="COLOR: #000000">=</span><span style="COLOR: #000000"> textLength</span><span style="COLOR: #000000">+</span><span style="COLOR: #000000">2</span><span style="COLOR: #000000">; </span><span style="COLOR: #008000">//</span><span style="COLOR: #008000">add space for null terminator</span><span style="COLOR: #008000">

             getText.flags = GT_DEFAULT;
getText.codepage = 1200; //Unicode

StringBuilder sb = new StringBuilder(getText.cb);
         SendMessage(</span><span style="COLOR: #0000ff">base</span><span style="COLOR: #000000">.Handle, 
                EM_GETTEXTEX, 
                </span><span style="COLOR: #0000ff">ref</span><span style="COLOR: #000000"> getText, sb);
     
          </span><span style="COLOR: #0000ff">return</span><span style="COLOR: #000000"> sb.ToString();
     }
     </span><span style="COLOR: #0000ff">set</span><span style="COLOR: #000000">
     {
         </span><span style="COLOR: #0000ff">base</span><span style="COLOR: #000000">.Text </span><span style="COLOR: #000000">=</span><span style="COLOR: #000000"> value;
     }
 }

 </span><span style="COLOR: #0000ff">public</span><span style="COLOR: #000000"> </span><span style="COLOR: #0000ff">override</span><span style="COLOR: #000000"> </span><span style="COLOR: #0000ff">int</span><span style="COLOR: #000000"> TextLength
 {
     </span><span style="COLOR: #0000ff">get</span><span style="COLOR: #000000">
     {
         GETTEXTLENGTHEX getLength </span><span style="COLOR: #000000">=</span><span style="COLOR: #000000"> </span><span style="COLOR: #0000ff">new</span><span style="COLOR: #000000"> GETTEXTLENGTHEX();
         getLength.flags </span><span style="COLOR: #000000">=</span><span style="COLOR: #000000"> GTL_DEFAULT; </span><span style="COLOR: #008000">//</span><span style="COLOR: #008000">Returns the number of characters</span><span style="COLOR: #008000">

             getLength.codepage = 1200; //Unicode

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

}
}
}
}