richtextbox自动滚动的问题
自动滚动已经搞定。但现在的问题是，加载的文档越长，滚动越快。怎么让它无论加载的文档多长。都保持给定的速度。

 private int min, max;
  private int pos = 0;
  private int endPos = 0;
  private const int SB_HORZ = 0x0;
  private const int SB_VERT = 0x1;
  private const int WM_HSCROLL = 0x114;
  private const int WM_VSCROLL = 0x115;
  private const int SB_THUMBPOSITION = 4;
  [DllImport("user32.dll")]
  private static extern int SetScrollPos(IntPtr hwnd, int nBar, int nPos, bool bRedraw);

  [DllImport("user32.dll")]
  private static extern int GetScrollPos(IntPtr hwnd, int nBar);
  [DllImport("user32.dll")]
  private static extern bool PostMessage(IntPtr hWnd, int nBar, int wParam, int lParam);
  [DllImport("user32", CharSet = CharSet.Auto)]
  private static extern bool GetScrollRange(IntPtr hWnd, int nBar, out int lpMinPos, out int lpMaxPos);
  private void rt_DoubleClick(object sender, EventArgs e)
  {
  timeAutoScroll.Interval = 100;
  timeAutoScroll.Interval -= t.AutoScrollSpeed;
  //得到滚动条的最大最小值 
  GetScrollRange(rt.Handle, SB_VERT, out min, out max);
  //得到滚动条到最底下的实际位置 
  endPos = max - rt.ClientRectangle.Height;
  this.timeAutoScroll.Enabled = true;

  }

  private void timeAutoScroll_Tick(object sender, EventArgs e)
  {
  pos = GetScrollPos(rt.Handle, SB_VERT);//加上这句是为了如果用户手动拖拽滚动条，可以保证滚动条继续从拖拽的位置走 
  pos++;
  //如果已经到底，那么停止Timer 
  if (pos > endPos)
  {
  this.timeAutoScroll.Enabled = false;
  return;
  }
  SetScrollPos(rt.Handle, SB_VERT, pos, true);
  PostMessage(rt.Handle, WM_VSCROLL, SB_THUMBPOSITION + 0x10000 * pos, 0);
  //PostMessage(rt.Handle, WM_VSCROLL, SB_THUMBPOSITION + pos*100000, 0);
  }

------解决方案--------------------
protected override void WndProc(ref Message m)
{
int n = 20;//该值越大，滑轮滚动速度超快

case WM_MOUSEWHEEL:
中滚动，控制pos-=n;

}