How to modify the appconfig file custom tag values?

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VB

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i am using vb.net for windows application.
i want o know to update the custom tag in config file.
in app config file initially i stored empty string. after my application run time i will replace the empty string into some value.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="tagx" type="System.Configuration.NameValueSectionHandler" />
  </configSections>
  <tagx>
    <add key="key1" value=""/>
  </tagx>
</configuration>

after run my application i want to update the key1 value as "KING".

i searched in net, all the examples giving how to update the appsettings tag value only.
i NO need appsettings updation. i want to update custom tag value.

thanks.

What I have tried:

Dim configFile = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None)
          Dim settings = CType(configFile.GetSection("tagx"), AppSettingsSection).Settings
          settings("key1").Value = "KING"
          configFile.Save(ConfigurationSaveMode.Modified)           ConfigurationManager.RefreshSection(configFile.AppSettings.SectionInformation.Name)

but it raise the exception in

Dim settings = CType(configFile.GetSection("tagx"), AppSettingsSection).Settings

unable to cast defaultsettings into appsettings