VB.NET要怎样才能读取和写入txt里面的对应位置

Using sw As New System.IO.StreamWriter(txt路径字符串)
Using sr As New System.IO.StreamReader(txt路径字符串)
Dim line As String
While sr.Peek() >= 0
line = sr.ReadLine()
If line.StartsWith("#a") Then
sw.WriteLine("#a 600")
Else
sw.WriteLine(line)
End If
End While
sr.Close()
End Using
sw.Flush()
End Using

手机写的难免有错见谅

Dim txt As String = My.Computer.FileSystem.ReadAllText("d:\123.txt", System.Text.Encoding.Default)
Dim s() As String = txt.Split(vbCr)
IO.File.WriteAllText("d:\123.txt", txt.Replace(s(1).Replace(vbLf, ""), "#a " & "新数值"), System.Text.Encoding.Default)
'#a为第2排，在数组中的索引为1，其他同，文本以记事本的标准换行。