reading a text file to an array - problem

[shadowfyre]

This seems so easy, but I'm pulling my hair out because I can't figure it out. Maybe some fresh eyes will be able to spot my glaringly simple mistake that I just can't find.

I want to read a text file in, line by line, sort the lines, and write back to the same text file. But I'm having problems assigning the line that is read from the file to an element of the array. Here's my code:

If File.Exists(outputpath & dir.ToLower & ".txt") Then
                'append to file
                sr = File.AppendText(outputpath & dir.ToLower & ".txt")
                sr.WriteLine(filename & ";" & filedate & ";" & dir & ";" & filesize)
                sr.Close()
                ' Sort file by filename
                Dim sortfile() As String
                Dim inputfile As New StreamReader(outputpath & dir.ToLower & ".txt")
                Dim inputstring As String
                inputstring = inputfile.ReadLine()
                Dim i As Integer = 0
                While (inputstring <> Nothing)
                    Try
                        sortfile(i) = inputstring
                    Catch ex As Exception
                        MessageBox.Show(ex.Message, "Caught Exception")
                    End Try
 
                    i = i + 1
                    inputstring = inputfile.ReadLine()
                End While
                inputfile.Close()
                System.Array.Sort(sortfile)
                Dim outputfile As New StreamWriter(outputpath & dir.ToLower & ".txt")
                Dim j As Integer = 0
                While j < sortfile.Length
                    outputfile.WriteLine(sortfile(j))
                    j = j + 1
                End While
                outputfile.Close()
            Else
                'create new file
                sr = File.CreateText(outputpath & dir.ToLower & ".txt")
                sr.WriteLine(filename & ";" & filedate & ";" & dir & ";" & filesize)
                sr.Close()

I keep getting the exception

Object reference not set to an instance of an object

when reading the file in, right at the line:

sortfile(i) = inputstring

Is this not allowed?