VB Remove blocks of text in textfile - vb.net

Question

Recently switched to vb after my time in C# and simple question using sytemIO. My predecessor wrote a package that generated error logs to a text file. The following is a sample:
2017-10-20 15:30:11.481
CmsMonitorService.exe, CmsMonitorService.UpdateCmsOffLine
OffLineUpdater error: Getting list of files stored on the off-line vault.
------------------------------
2017-10-20 15:31:11.547
CmsMonitorService.exe, CmsMonitorService.UpdateCmsOffLine
OffLineUpdater error: Creating folder 'OffLineUpdates' (it may already exist checkHost)
at CmsMonitorService.CmsMonitorService.UpdateCmsOffLine(Object[] Args)
------------------------------
2017-10-20 15:31:11.547
CmsMonitorService.exe, CmsMonitorService.UpdateCmsOffLine
OffLineUpdater error: Creating folder
------------------------------
But this is killing the machines. What the code currently does when it is writing is to removed the contents line by line which is painfully slow. It uses the following:
Do
If allLines.Count = 0 Then
Exit Do
ElseIf allLines(0).StartsWith("-----") Then
allLines.RemoveAt(0)
Exit Do
Else
allLines.RemoveAt(0)
End If
Loop
There can be thousands of these (they are at various locations).
What I had wanted to do is find a way of removing the blocks bewteen the dashes.
Thanks for any ideas everyone.....
Gareth

Answer 1

Here's a quick example of the solution I described in my comment above:
Imports System.Text
Module Module1
Sub Main()
Dim s = "keep line 1" & Environment.NewLine &
"keep line 2" & Environment.NewLine &
"----------" & Environment.NewLine &
"remove line 1" & Environment.NewLine &
"remove line 2" & Environment.NewLine &
"----------" & Environment.NewLine &
"keep line 3" & Environment.NewLine &
"keep line 4" & Environment.NewLine &
"----------" & Environment.NewLine &
"remove line 3" & Environment.NewLine &
"remove line 4" & Environment.NewLine &
"----------" & Environment.NewLine &
"keep line 5" & Environment.NewLine &
"keep line 6" & Environment.NewLine
Dim sb As New StringBuilder(s)
Dim endIndex = s.LastIndexOf("----------")
Do While endIndex <> -1
Dim startIndex = s.LastIndexOf("----------", endIndex - 1)
Dim substring = s.Substring(startIndex, endIndex - startIndex + 12) 'Add the length of the delimiter and the line break.
'Remove the delimited block from the StringBuilder.
sb.Replace(substring, String.Empty, startIndex, substring.Length)
endIndex = s.LastIndexOf("----------", startIndex - 1)
Loop
Console.WriteLine("Before:")
Console.WriteLine(s)
Console.WriteLine("After:")
Console.WriteLine(sb.ToString())
Console.ReadLine()
End Sub
End Module
It might depend on the specifics of the text as to whether that is more efficient or just using a String alone is.