Reading Text line by line to make string manipulation - vb.net

Question

So to start this is the code I have already written:
Dim MyFile As String = "Path"
Dim str_new As String
Dim str_old As String = File.ReadAllText(MyFile)
Dim sr As New StreamReader(MyFile)
Dim strLines As String() = Strings.Split(sr.ReadToEnd, Environment.NewLine)
Dim Character As Integer = 5 'Line 1 always has 5 characters
For i = 2 To strLines.Length
Dim PreviousLine As String = sr.ReadLine(i - 1)
Dim CurrentLine As String = sr.ReadLine(i)
If CurrentLine.Contains(TextBox1.Text / 100) Then
If PreviousLine.Contains("divide") Then
Exit For
End If
End If
Character = Character + CurrentLine.Length
Next
sr.Close()
str_new = Replace(str_old, (TextBox1.Text / 100), (TextBox3.Text / 100), Character, 1)
Dim objWriter3 As New System.IO.StreamWriter(MyFile, False)
objWriter3.Write(str_new)
objWriter3.Flush()
objWriter3.Close()
I am trying to figure out a way to break a long code file into lines then check each line for certain strings. If the current line contains the string then I will do additional check on above and/or below lines to make sure This is the correct instance of the string. Finally I want to replace just that instance of the string with a different string.
An example: text file
class
...
0.3
divide <-- Previous Line
0.3 <-- TextBox1.Text is 30
.5
end
I want the code to go past the first instance of 0.3
Find the second instance
Check previous line for divide
Exit Loop
Replace second instance of 0.3 to some value
I have been looking into this for a while now and any help would be greatly appreciated!
~Matt
Revised: Code
Dim MyFile As String = "Path"
Dim NewFile As String = "Temporary Path"
Dim PreviousLine As String = ""
Dim CurrentLine As String = ""
Using sr As StreamReader = New StreamReader(MyFile)
Using sw As StreamWriter = New StreamWriter(NewFile)
CurrentLine = sr.ReadLine
Do While (Not CurrentLine Is Nothing)
Dim LinetoWrite = CurrentLine
If CurrentLine.Contains(TextBox1.Text) Then
If PreviousLine.Contains("divide") Then
LinetoWrite = Replace(CurrentLine, TextBox1.Text, TextBox3.Text)
End If
End If
sw.WriteLine(LinetoWrite)
PreviousLine = CurrentLine
CurrentLine = sr.ReadLine
Loop
End Using
End Using
My.Computer.FileSystem.CopyFile(NewFile, MyFile, True)

Answer 1

You are facing the problem in the wrong way. You have to set both, reader and writer, and generate a new file with all the modifications. Your code has various parts which should be improved; in this answer, I am just showing how to use StreamReader/StreamWriter properly. Sample code:
Dim MyFile As String = "input path"
Dim OutputFile As String = "output path"
Dim PreviousLine As String = ""
Dim CurrentLine As String = ""
Using sr As StreamReader = New StreamReader(MyFile)
Using sw As StreamWriter = New StreamWriter(OutputFile)
CurrentLine = sr.ReadLine
Do While (Not CurrentLine Is Nothing)
Dim lineToWrite = CurrentLine
'Perform the analysis you wish by involving the current line, the previous one and as many other (previous) lines as you wish; and store the changes in lineToWrite. You should call a function here to perform this analysis
sw.WriteLine(lineToWrite) 'Writing lineToWrite to the new file
PreviousLine = CurrentLine 'Future previous line
CurrentLine = sr.ReadLine 'Reading the line for the next iteration
Loop
End Using
End Using