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
    {
        // RichEdit messages (Richedit.h) 
        private const int EM_GETTEXTEX = (WM_USER + 94);
        private const int EM_GETTEXTLENGTHEX = (WM_USER + 95);
        // Flags for the GETEXTEX data structure 
        private const int GT_DEFAULT = 0;
        // Flags for the GETTEXTLENGTHEX data structure 
        // Fast computation of a "close" answer 
        private const int GTL_CLOSE = 4;
        // Do default (return # of chars) 
        private const int GTL_DEFAULT = 0; 
        private const int WM_USER = 0x0400;

        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();
                //add space for null terminator 
                getText.cb = textLength + 2; 
                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();
                //Returns the number of characters 
                getLength.flags = GTL_DEFAULT; 
                getLength.codepage = 1200; //Unicode 
                return SendMessage(base.Handle, EM_GETTEXTLENGTHEX, ref getLength, 0);
            }
        }

        [DllImport("user32.dll", EntryPoint = "SendMessage", CharSet = CharSet.Auto)]
        private static extern int SendMessage(IntPtr hWnd, int msg,
                                              ref GETTEXTEX wParam, StringBuilder lParam);

        [DllImport("user32.dll", EntryPoint = "SendMessage", CharSet = CharSet.Auto)]
        private static extern int SendMessage(IntPtr hWnd, int msg,
                                              ref GETTEXTLENGTHEX wParam, int lParam);

        #region Nested type: GETTEXTEX

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

        #endregion

        #region Nested type: GETTEXTLENGTHEX

        [StructLayout(LayoutKind.Sequential)]
        private struct GETTEXTLENGTHEX
        {
            public Int32 flags;
            public Int32 codepage;
        }

        #endregion
    }
}