Question

I am currently working on a method that takes in a text file and will reduce the file down to ~10 MB. This method is used to truncate log files and keep them within a 10 MB limit.

The logic behind the code is basically this… if the file is 250 MB or bigger then read the bytes till the array reaches 250 MB. Store this into a StringBuilder, set position for next read and repeat until the StringBuilder contains ~10 MB of data. Then write out to the file erasing all the data and leaving only 10 MB of the most recent writes.

To prevent cutting lines in half, it checks to see where the last CrLf is and then writes out all the data from that point forward.

My problem is I can’t get the seek to correctly position itself after the first read. It reads the data correctly first go, then when I use that position from the previous read for the next iteration it “ignores” the position and reads from the beginning of the file again.

If logFile.Length > (1024 * 1024 * 250) Then
    Dim DataToDelete As Integer = logFile.Length - (1024 * 1024 * 250)
    Dim ArrayIndex As Integer = 0
    While DataToDelete > 0
        Using fs As FileStream = New FileStream(logFile.FullName, FileMode.Open, FileAccess.ReadWrite)
            fs.Seek(ArrayIndex, SeekOrigin.Begin)
            If strBuilder.Length < (1024 * 1024 * 250) Then
                Dim bytes() As Byte = New Byte((1024 * 1024 * 250)) {}
                Dim n As Integer = fs.Read(bytes, 0, (1024 * 1024 * 250))
                ArrayIndex = bytes.Length
                Dim enc As Encoding = Encoding.UTF8
                strBuilder.Append(enc.GetString(bytes))
            Else
                If DataToDelete - strBuilder.Length < 0 And strBuilder.Length > (1024 * 1024 * My.Settings.Threshold) Then
                    Dim DataToCut As Integer = strBuilder.Length - (1024 * 1024 * My.Settings.Threshold)
                    While Not (strBuilder.Chars(DataToCut).ToString.Equals(vbCr)) And DataToCut <> 0
                        DataToCut -= 1
                    End While
                    strBuilder.Remove(0, DataToCut)
                    File.WriteAllText(logFile.FullName, strBuilder.ToString)
                Else
                    DataToDelete -= strBuilder.Length
                    strBuilder.Clear()
                End If
            End If
        End Using
    End While
End If

Answer 1

This is my end result,works like a charm!

        Dim Maxsize As Integer = (1024 * 1024 * My.Settings.Threshold)
    For Each logfile In filesToTrim
        Dim sb As New StringBuilder
        Dim buffer As String = String.Empty
        If logfile.Length > Maxsize Then
            Using reader As New StreamReader(logfile.FullName)
                reader.BaseStream.Seek(-Maxsize, SeekOrigin.End)
                buffer = reader.ReadToEnd()
                sb.Append(buffer)
            End Using
            Dim Midpoint As Integer = 0
            While Not (sb.Chars(Midpoint).ToString.Equals(vbCr)) And Midpoint <> sb.Length - 1
                Midpoint += 1
            End While
            sb.Remove(0, Midpoint)
            File.WriteAllText(logfile.FullName, sb.ToString)
        End If
    Next