IOException was unhandled "The process cannot access the file - vb.net

Question

I am trying to reading the file, reformat some rows, then overwrite the file line by line.
I know the problem is cause by opening the existing file twice, but I don't know how to fix it.
Public Sub Main()
Dim objStreamReader As StreamReader
Dim objWriter As StreamWriter
If Dts.Variables("File").Value.ToString.Length > 0 Then
'read the file
objStreamReader = New StreamReader(Dts.Variables("File").Value.ToString)
'write the file
objWriter = New StreamWriter(Dts.Variables("File").Value.ToString)
'read file line by line
Dim row, line As String
Dim isFirstRow As Boolean = True
Dim numColumns As Integer = 0
row = ""
Do While objStreamReader.Peek() <> -1
line = objStreamReader.ReadLine() 'read a line
If line.Trim() = "" Then
Continue Do 'skip blank line
End If
If isFirstRow Then 'first line in the file (it's always a complete row)
numColumns = CountNumColumns(line) 'record the number of columns
isFirstRow = False
objWriter.WriteLine(line)
ElseIf CountNumColumns(line) = numColumns Then
'the line is a complete row
objWriter.WriteLine(line)
Else
row = row & line
If CountNumColumns(row) = numColumns Then
'we have a complete row
objWriter.WriteLine(row)
row = ""
End If
End If
Loop
'close files
objStreamReader.Close()
objWriter.Close()
End If
Dts.TaskResult = ScriptResults.Success
End Sub
'Counts the number of columns in a given line
Function CountNumColumns(ByVal line As String) As Integer
Dim count As Integer = 0
For index As Integer = 0 To line.Length - 1
If line(index) = "|" Then
count = count + 1
End If
Next
Return count
End Function