Find word in a txt file and read previous line VB.NET - vb.net

Question

I am reading a txt file line by line in VB to look for the word "unable". That much works. The code is here:
Imports System
Imports System.IO
Imports PartMountCollector.HandMount_WebReference
Imports System.Threadingtime
Imports eCenter.Motor.VBConnect
Module Program
Sub Main(args As String())
Dim unUpdate As String = "Unable"
Dim time = DateTime.Now
Dim yesterday = time.AddDays(-1)
Dim format As String = "yyyyMMdd"
Dim words As String()
For Each Line As String In File.ReadLines("C:\Users\te-smtinternal\Desktop\ReStockLog\" + time.ToString(format) + ".txt")
words = Split(Line)
If Line.Contains(unUpdate) = True Then
Console.WriteLine("Exist")
'Read previous line looking for "Success"'
End If
Console.WriteLine("not found")
Next
End Sub
End Module
Now I need be able to identify this line and read the previous line, looking for the word "success".
Any help would be appreciated

Answer 1

Instead of trying to handle each line one at a time, you could read all the lines and then iterate through them which would give you access to the previous line, like:
Dim lines = IO.File.ReadAllLines(file_name)
dim previous_line as string = ""
for x as integer = 0 to lines.count-1
if lines(x).ToString.Contains("unable") then previous_line = lines(x-1).ToString
next
of course you would need to handle the exception of finding a hit on the first line which would throw an out of index error. So you would simply need to add a check to make sure x > 0.

Answer 2

You just need to declare a variable outside of the loop to store the previous line. Here I've named it previousLine...
Const unUpdate As String = "Unable"
Dim time = DateTime.Now
Const format As String = "yyyyMMdd"
Dim previousLine as String = Nothing
For Each currentLine As String In File.ReadLines("C:\Users\te-smtinternal\Desktop\ReStockLog\" + time.ToString(format) + ".txt")
If currentLine.Contains(unUpdate) Then
Console.WriteLine("Exist")
If previousLine Is Nothing Then
' The very first line of input contains unUpdate
Else If previousLine.Contains("Success")
' A line after the first line of input contains unUpdate
End If
Else
Console.WriteLine("not found")
End If
previousLine = currentLine
Next
At the end of each loop iteration the currentLine becomes the previousLine and, if there is another iteration, it will read a new value for currentLine.
Also note that in...
If Line.Contains(unUpdate) = True Then
...you don't need the = True comparison because Contains() already returns a Boolean.