private static extern bool GetScrollInfo(IntPtr hwnd, int fnBar, ref SCROLLINFO lpsi);

Public Function GetScrollInfo(hWnd As IntPtr, _

Declare Function GetScrollInfo Lib "user32" (ByVal hWnd As IntPtr, ByVal fnBar As ScrollBarDirection, ByRef lpsi As SCROLLINFO) As Integer

      GetScrollInfo( RichTextBox1.Handle, (int) ScrollBarDirection.SB_HORZ, ref si );

    Dim lRet As Integer = GetScrollInfo(hWnd, fnBar, si)