Writing Part/Block of File to another File - vb.net

Question

Please Help: I would like to read one block from file inPath and write to another file outPath.
I am using ReadLines method to read the File line by line and when reach at START_BLOCK, start writing to the output file and continue until you find the END_BLOCK.
I know couple of other methods by copying the whole file into a variable and pick the block what I need. I can't use saving on a variable as my files are very big GB+
The coding I have below copies the line at the "START_BLOCK" Can't really figure it out how to continue writing until the "END_BLOCK". Please suggest and thank you very much in advance.
Dim inPath As String = "C:\temprm\myFile.txt"
Dim outPath As String = "C:\temprm\myFileNew1.txt"
Using sw As StreamWriter = File.CreateText(outPath)
For Each line As String In File.ReadLines(inPath)
If line.Contains("START_BLOCK") Then
sw.WriteLine(line)
'-------HOW DO I CONTINUE TO WRITE UNTIL "END_BLOCK"
End If
Next line
End Using

Answer 1

You could just set a flag to indicate that you are inside of the block, and use that to write out the lines until you find the end tag, e.g. something like this (untested code!):
Dim inPath As String = "C:\temprm\myFile.txt"
Dim outPath As String = "C:\temprm\myFileNew1.txt"
Dim insideBlock As Boolean = False
Using sw As StreamWriter = File.CreateText(outPath)
For Each line As String In File.ReadLines(inPath)
If line.Contains("START_BLOCK") Then
sw.WriteLine(line)
insideBlock = True
ElseIf line.Contains("END_BLOCK") Then
sw.WriteLine(line)
insideBlock = False
Exit For
ElseIf insideBlock Then
sw.WriteLine(line)
End If
Next line
End Using
UPDATE
Since the comments are getting out of control - here's a version to handle multiple blocks with different start tags but the same end tag (untested since I'm at home on my Mac):
Dim inPath As String = "C:\temprm\myFile.txt"
Dim outPath As String = "C:\temprm\myFileNew1.txt"
Dim insideBlock As Boolean = False
Using sw As StreamWriter = File.CreateText(outPath)
For Each line As String In File.ReadLines(inPath)
If IsStartOfBlock(line) Then
sw.WriteLine(line)
insideBlock = True
ElseIf line.Contains("END_BLOCK") Then
sw.WriteLine(line)
insideBlock = False
ElseIf insideBlock Then
sw.WriteLine(line)
End If
Next line
End Using
'...
' Logic to determine if the line is the start of a block, for example:
Private Function IsStartOfBlock(line As String) As Boolean
Dim startMarkers() As String = {
"START_BLOCK", "START_BLOCK2", "START_BLOCKX"
}
Return startMarkers.Any(Function(x) line.Contains(x))
End Function
The loop will exit at the end of file anyway, so the last block should be ok too.