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[QUOTE="jmcilhinney, post: 186087, member: 641"]
I haven't read your code in detail but, at a glance, it looks very inefficient. For one thing, you should not be using the state of UI controls to determine the state of reading a file. Just read each file line by line and write out the data line by line.
[CODE=vbnet]Sub Main()
    Using sourceReader1 As New StreamReader("source file 1 path here"),
          sourceReader2 As New StreamReader("source file 2 path here"),
          destinationWriter As New StreamWriter("destination file path here")
        Do Until sourceReader1.EndOfStream AndAlso sourceReader2.EndOfStream
            If Not sourceReader1.EndOfStream Then
                TransferData(sourceReader1, destinationWriter)
            End If

If Not sourceReader2.EndOfStream Then
                TransferData(sourceReader2, destinationWriter)
            End If
        Loop
    End Using
End Sub

Private Sub TransferData(sourceReader As StreamReader, destinationWriter As StreamWriter)
    Const TARGET_LINE As String = "target line here"

Dim line As String

Do
        line = sourceReader.ReadLine()

destinationWriter.WriteLine(line)
    Loop Until line = TARGET_LINE OrElse sourceReader.EndOfStream
End Sub[/CODE]
It's still going to take some time if there is a lot of data but it's more straightforward than what you're doing. You may need to adjust it a bit for your specific needs.
[/QUOTE]

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