Get current scroll position from rich text box control?

Question

I have searched the internet far and wide and seen many questions like this, but I have not seen an actual answer.

I have a rich text box control with lots of text in it. It has some legal information in this control. By default the "Accept" button is disabled. I want to detect on the scroll event if the position of the v-scroll bar is at the bottom. If it is at the bottom, enable the button.

How would I detect the current v-scroll bar position?

Thank You!

EDIT I am using WinForms (.Net 4.0)

Answer 1

This should get you close to what you are looking for. This class inherits from the RichTextBox and uses some pinvoking to determine the scroll position. It adds an event ScrolledToBottom which gets fired if the user scrolls using the scrollbar or uses the keyboard.

public class RTFScrolledBottom : RichTextBox {
  public event EventHandler ScrolledToBottom;

  private const int WM_VSCROLL = 0x115;
  private const int WM_MOUSEWHEEL = 0x20A;
  private const int WM_USER = 0x400;
  private const int SB_VERT = 1;
  private const int EM_SETSCROLLPOS = WM_USER + 222;
  private const int EM_GETSCROLLPOS = WM_USER + 221;

  [DllImport("user32.dll")]
  private static extern bool GetScrollRange(IntPtr hWnd, int nBar, out int lpMinPos, out int lpMaxPos);

  [DllImport("user32.dll")]
  private static extern IntPtr SendMessage(IntPtr hWnd, Int32 wMsg, Int32 wParam, ref Point lParam);

  public bool IsAtMaxScroll() {
    int minScroll;
    int maxScroll;
    GetScrollRange(this.Handle, SB_VERT, out minScroll, out maxScroll);
    Point rtfPoint = Point.Empty;
    SendMessage(this.Handle, EM_GETSCROLLPOS, 0, ref rtfPoint);

    return (rtfPoint.Y + this.ClientSize.Height >= maxScroll);
  }

  protected virtual void OnScrolledToBottom(EventArgs e) {
    if (ScrolledToBottom != null)
      ScrolledToBottom(this, e);
  }

  protected override void OnKeyUp(KeyEventArgs e) {
    if (IsAtMaxScroll())
      OnScrolledToBottom(EventArgs.Empty);

    base.OnKeyUp(e);
  }

  protected override void WndProc(ref Message m) {
    if (m.Msg == WM_VSCROLL || m.Msg == WM_MOUSEWHEEL) {
      if (IsAtMaxScroll())
        OnScrolledToBottom(EventArgs.Empty);
    }

    base.WndProc(ref m);
  }

}

This is then how it can get used:

public Form1() {
  InitializeComponent();
  rtfScrolledBottom1.ScrolledToBottom += rtfScrolledBottom1_ScrolledToBottom;
}

private void rtfScrolledBottom1_ScrolledToBottom(object sender, EventArgs e) {
  acceptButton.Enabled = true;
}

Tweak as necessary.