Catch the RichTextBox's EN_REQUESTRESIZE notifications and modify the
controls bounds according to the information returned in the REQUESTSIZE
structure.

Simple example of autosizing a RichTextBox follows, based upon a standard
RichTextBox with both BorderStyle and Scrollbars set to none.

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public partial class Form1 : Form
{
   public Form1()
   {
       InitializeComponent();
       txtWnd = new NativeTextBox();
       txtWnd.AssignHandle(this.richTextBox1.Handle);

//Simple method to force a Resize on an empty richtextbox
       this.richTextBox1.Text = "a";
       this.richTextBox1.Text = "";
   }

   public NativeTextBox txtWnd;

   [StructLayout(LayoutKind.Sequential)]
   public struct NMHDR
   {
       public IntPtr HWND;
       public uint idFrom;
       public int code;
       public override String ToString()
       {
           return String.Format("Hwnd: {0}, ControlID: {1}, Code: {2}",
HWND, idFrom, code);
       }
   }

   [StructLayout(LayoutKind.Sequential)]
   public struct REQRESIZE
   {
       public NMHDR nmhdr;
       public RECT rc;
   }

   [StructLayout(LayoutKind.Sequential)]
   public struct RECT
   {
       public int Left, Top, Right, Bottom;
       public override string ToString()
       {
           return String.Format("{0}, {1}, {2}, {3}", Left, Top, Right,
Bottom);
       }
       public Rectangle ToRectangle()
       {
           return Rectangle.FromLTRB(Left, Top, Right, Bottom);
       }
   }

   public const int WM_USER = 0x400;
   public const int WM_NOTIFY = 0x4E;
   public const int WM_REFLECT = WM_USER + 0x1C00;
   public const int EN_REQUESTRESIZE = 0x701;

   public class NativeTextBox : NativeWindow
   {
       protected override void WndProc(ref Message m)
       {
           base.WndProc(ref m);
           if (m.Msg == (WM_REFLECT | WM_NOTIFY))
           {
               NMHDR hdr = (NMHDR)(Marshal.PtrToStructure(m.LParam,
typeof(NMHDR)));
               if (hdr.code == EN_REQUESTRESIZE)
               {
                   REQRESIZE rrs =
(REQRESIZE)(Marshal.PtrToStructure(m.LParam, typeof(REQRESIZE)));
                   Control tb = Control.FromHandle(this.Handle);
                   Rectangle rc =rrs.rc.ToRectangle();
                   tb.SetBounds(0,0,tb.Width,rc.Height,
BoundsSpecified.Size);
               }
           }
       }
   }
}
///