How to replace text string in a text file - vb.net

Question

I am new to vb.net and am trying to do something really simple. I have this code which reads certain line of text from .ini file.
Dim FilePath As String = Application.StartupPath & "\bin\userconfig.ini"
Dim text As String = IO.File.ReadAllText(FilePath)
Dim newText = text.Replace("UserName = ", TextBox_NewUser.Text)
IO.File.WriteAllText(FilePath, newText)
How do I make it replace that line of text after the "=" with something you type in TextBox_NewUser. As you can see with current code it just replaces the whole "UserName =" which I don't want.
That specific line of text in the .ini by default has this value:
"UserName = Unnamed"
So how do I make it replace just that "Unnamed" with something I type in TextBox_NewUser?
Any assistance will be most appreciated.

Answer 1

Dim newText = text.Replace("UserName = Unnamed", "UserName = " & TextBox_NewUser.Text)

Answer 2

Here is another way to go about this, there are additional assertions that could be done e.g. the code below assumes the lines don't begin with spaces and if they did you would first do a trim on each line before using StartsWith etc.
Config file
Role = Moderator
UserName = xxxx
Joined = 09/23/1006
Code
Public Class Form1
Private fileName As String =
IO.Path.Combine(
AppDomain.CurrentDomain.BaseDirectory, "userConfig.ini")
''' <summary>
''' assumes the file exist but does not assume there is text in
''' the text box.
''' </summary>
''' <param name="sender"></param>
''' <param name="e"></param>
''' <remarks></remarks>
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
If Not String.IsNullOrWhiteSpace(TextBox_NewUser.Text) Then
Dim lines As List(Of String) = IO.File.ReadAllLines(fileName).ToList
For index As Integer = 0 To lines.Count - 1
If lines(index).ToLower.StartsWith("username") Then
lines(index) = String.Concat("UserName = ", TextBox_NewUser.Text)
End If
Next
IO.File.WriteAllLines(fileName, lines.ToArray)
End If
End Sub
End Class
Sample project on Microsoft OneDrive, VS2013
https://onedrive.live.com/redir?resid=A3D5A9A9A28080D1!907&authkey=!AKQCanSTiCLP4mE&ithint=file%2czip