how can i update app.config file with connection string at run time in vb.net

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VB

i have this application and i want user to decide which database they want to connect to at run time by supplying the data source,user id and password of the database before the application will run. i am using vb.net with oracle.any input will be appreciated. thanks. The following codes is what I have done but it does not seem to get me the result I want.

Public Sub savenewconnection(ByVal id As String, ByVal pass As String, ByVal source As String)
       Try
           Dim newdbcon As String
           Dim oldconnection As ConnectionStringSettings = ConfigurationManager.ConnectionStrings("mydbcon")
           Dim dbcon As String = oldconnection.ConnectionString
           Dim builder As New OleDbConnectionStringBuilder(dbcon)
           builder.Add("Data Source", source)
           builder.Add("Password", pass)
           builder.Add("User ID", id)
           newdbcon = builder.ConnectionString
           Dim config As System.Configuration.Configuration = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None)
           config.ConnectionStrings.ConnectionStrings("mydbcon").ConnectionString = newdbcon
           config.Save(ConfigurationSaveMode.Modified)
           ConfigurationManager.RefreshSection("ConnectionStrings")

       Catch ex As Exception
           MsgBox(ex.Message)
       End Try
   End Sub