Windows Forms

Dim i As Integer = TextBox1.SelectionStart
Dim atEnd As Boolean = i >= TextBox1.Text.Length

TextBox1.AppendText("New line: " + TextBox1.Text.Length.ToString() + vbNewLine)

If Not atEnd Then
    TextBox1.SelectionStart = i
    TextBox1.ScrollToCaret()

End If

    public partial class PeerMessagesDisplay : Form
    {
        #region ScrollInfo

        [DllImport( "user32.dll" )]
        [return: MarshalAs( UnmanagedType.Bool )]
        private static extern bool GetScrollInfo( IntPtr hwnd, int fnBar, ref SCROLLINFO lpsi );

        [DllImport( "user32.dll" )]
        static extern int SetScrollInfo( IntPtr hwnd, int fnBar, [In] ref SCROLLINFO
            lpsi, bool fRedraw );

        [StructLayout( LayoutKind.Sequential )]
        struct SCROLLINFO
        {
            public uint cbSize;
            public uint fMask;
            public int nMin;
            public int nMax;
            public uint nPage;
            public int nPos;
            public int nTrackPos;
        }

        private enum ScrollBarDirection
        {
            SB_HORZ = 0,
            SB_VERT = 1,
            SB_CTL = 2,
            SB_BOTH = 3
        }

        private enum ScrollInfoMask
        {
            SIF_RANGE = 0x1,
            SIF_PAGE = 0x2,
            SIF_POS = 0x4,
            SIF_DISABLENOSCROLL = 0x8,
            SIF_TRACKPOS = 0x10,
            SIF_ALL = SIF_RANGE + SIF_PAGE + SIF_POS + SIF_TRACKPOS
        }

        #endregion

        private String title;
        private String logFile;

        private bool disposed;

        public PeerMessagesDisplay( String title, String logFilePath )
        {
            InitializeComponent();

            this.title = title;
            this.logFile = logFilePath;

            this.Show();
        }


        public void LoadLog( )
        {
            ThreadPool.QueueUserWorkItem( delegate
            {
                try
                {
                    if (this.InvokeRequired)
                        this.BeginInvoke( new Action<string>( delegate( String s ) { this.Text = s; } ), title );
                    else
                        this.Text = title;

                    using (FileStream fs = File.OpenRead( this.logFile ))
                    {
                        using (StreamReader reader = new StreamReader( fs ))
                        {
                            String text = reader.ReadToEnd();
                            AddNewMessage( text );
                        }
                    }
                }
                catch (IOException ioe)
                {
                    AddNewMessage( ioe.Message );
                }
            } );
        }


        public void AddNewMessage( String message )
        {
            lock (this)
            {
                if (this.textBox1.InvokeRequired)
                    this.textBox1.BeginInvoke( new Action<string>( AddNewMessageInvoke ), message );
                else
                    AddNewMessageInvoke( message );
            }
        }

        private void AddNewMessageInvoke( String message )
        {
            SCROLLINFO scrollInfo = new SCROLLINFO();
            GetScrollInfo( this.textBox1.Handle, (int)ScrollBarDirection.SB_VERT, ref scrollInfo );
            
            this.textBox1.AppendText( message );

            SetScrollInfo( this.textBox1.Handle, (int)ScrollBarDirection.SB_VERT, ref scrollInfo, true );
        }
    }
}
</string></string>

scrollInfo.cbSize = Marshal.SizeOf(SCROLLINFO);
scrollInfo.fMask = ScrollInfoMask.SIF_POS;