p/invoke SendMessage to Control scroll bar, and implement a simple compare windows with RichTextBox

this.richTextBox1.VScroll += new System.EventHandler(this.richTextBox1_VScroll);
this.richTextBox2.VScroll += new System.EventHandler(this.richTextBox1_VScroll);

[DllImport("user32.dll")]
static extern int SendMessage(IntPtr hWnd, UInt32 Msg, Int32 wParam, Int32 lParam);
const int EM_GETFIRSTVISIBLELINE = 0x00CE;
const int EM_LINESCROLL = 0x00B6;
private void richTextBox1_VScroll(object sender, EventArgs e)
{
  int linenum1 = SendMessage(this.richTextBox1.Handle, EM_GETFIRSTVISIBLELINE, 0, 0);

  int linenum2 = SendMessage(this.richTextBox2.Handle, EM_GETFIRSTVISIBLELINE, 0, 0);

  int lines = linenum1 - linenum2;
  IntPtr Hrtb = this.richTextBox2.Handle;
  if(this.richTextBox2.Focused)
  {
    lines = - lines;
    Hrtb = this.richTextBox1.Handle;
  }

  SendMessage(Hrtb, EM_LINESCROLL, 0, lines);
}