'[Info.] The following codes show how to detect an event that 
' user scroll the scrollbars in a RichTextBox to the end
'
' *Required controls :
' 1x Form
' 1x RichTextBox named as "RichTextBox1"



Imports System.Runtime.InteropServices

Public Class Form1

    '========== API ==========

    <StructLayout(LayoutKind.Sequential)> Public Structure SCROLLINFO
        Public cbSize As Integer
        Public fMask As Integer
        Public nMin As Integer
        Public nMax As Integer
        Public nPage As Integer
        Public nPos As Integer
        Public nTrackPos As Integer
    End Structure

    Public Declare Function GetScrollInfo Lib "user32" Alias "GetScrollInfo" ( _
    ByVal hWnd As IntPtr, _
    ByVal nBar As Integer, _
    <MarshalAs(UnmanagedType.Struct)> ByRef lpScrollInfo As SCROLLINFO) As Integer

    Private Const SB_HORZ As Integer = 0
    Private Const SB_VERT As Integer = 1
    Private Const SIF_RANGE As Integer = &H1
    Private Const SIF_PAGE As Integer = &H2
    Private Const SIF_POS As Integer = &H4
    Private Const SIF_DISABLENOSCROLL As Integer = &H8
    Private Const SIF_TRACKPOS As Integer = &H10
    Private Const SIF_ALL As Integer = (SIF_RANGE Or SIF_PAGE Or SIF_POS Or SIF_TRACKPOS)


    '========== Records ==========

    Dim var_last_scrollbarRatio As Double 'Record last scrollbard ratio, used in 'check_scrollbar_ratio()' func.


    '========== Functions ==========

    '\brief     Get user-defined scrollbar info
    '\param     ByVal hwnd As System.IntPtr     |   Handle of object
    '           ByVal iMode As Integer          |   Mode (Integer)
    '           ByRef si As SCROLLINFO          |   'SCROLLINFO' structure
    '\return    Result of 'GetScrollInfo' API func.
    Private Function get_scrollbarInfo(ByVal hwnd As System.IntPtr, ByVal iMode As Integer, _
                                       ByRef si As SCROLLINFO) As Integer

        Dim iRst As Integer 'Result


        With si
            .fMask = SIF_ALL
            .cbSize = Len(si)

            iRst = GetScrollInfo(hwnd, iMode, si)
        End With

        Return iRst

    End Function


    '\brief     Get scrollbar value/max (Ratio)
    '\param     ByRef si As SCROLLINFO          |   'SCROLLINFO' structure
    '\return    Double type : 0 ~ 1, 0 as first line, 1 as last line
    Private Function get_scrollbarRatio(ByRef si As SCROLLINFO) As Double

        If 0 <> (si.nMax - si.nPage) Then
            Return si.nPos / (si.nMax - si.nPage)
        Else
            Return 0
        End If

    End Function


    '\brief     Check scrollbar ratio (User-defined)
    '
    Private Sub check_scrollbar_ratio()

        Dim si As SCROLLINFO
        Dim iRst As Integer
        Dim dRatio As Double


        iRst = get_scrollbarInfo(RichTextBox1.Handle, SB_VERT, si)
        dRatio = get_scrollbarRatio(si)
        '' Debug.Print("Scrollbar ratio : " & dRatio) '_DEBUG_

        'Ratio must exceed 1 and it's not equal to last recorded ratio
        ' and last recorded ratio is lower than 1
        ' (Means that there are two states : one is not scrolled to the end, and otherwise)
        If dRatio >= 1 And dRatio <> var_last_scrollbarRatio And var_last_scrollbarRatio < 1 Then
            MessageBox.Show("You have scrolled to last line !!", "Scrollbar said", _
                            MessageBoxButtons.OK, MessageBoxIcon.Information)
        End If

        'Record last ratio
        var_last_scrollbarRatio = dRatio

    End Sub


    '========== Object events ==========

    Private Sub RichTextBox1_VScroll(ByVal sender As Object, ByVal e As System.EventArgs) Handles RichTextBox1.VScroll

        check_scrollbar_ratio()

    End Sub

End Class