Write appSettings in external file

Question

I have a config file app.exe.config and appSettings section has something like this:

Answer 1

A more complete answer to prevent confusion:

Setup:

1. Commandline project called 'app'

2. app.exe.config file, App.config:

3. App.Settings.config file with 'Copy to Output Directory'= 'Copy Always'

4. Program.cs:

   static void Main(string[] args)
   {
       try
       {
           Console.WriteLine("Local Config sections");
           var exepath = (new Uri(System.Reflection.Assembly.GetExecutingAssembly().CodeBase)).LocalPath;
           Configuration config = ConfigurationManager.OpenExeConfiguration(exepath);
   
           config.AppSettings.SectionInformation.ConfigSource = "App.Settings.config";
   
           Console.WriteLine("BEFORE[test]=" + config.AppSettings.Settings["test"].Value);
           Console.WriteLine($"BEFORE[testExternalOnly]={config.AppSettings.Settings["testExternalOnly"]?.Value}");
   
           //to avoid: Error CS0266
           //Explicitly cast 'System.Configuration.AppSettingsSection'
           AppSettingsSection myAppSettings = (AppSettingsSection)config.GetSection("appSettings");
   
           myAppSettings.Settings["test"].Value = "NEW";
           if (!myAppSettings.Settings.AllKeys.Contains("testExternalOnly"))
               myAppSettings.Settings.Add("testExternalOnly", "NEWEXTERNAL");
   
           config.Save(ConfigurationSaveMode.Modified);
           ConfigurationManager.RefreshSection("appSettings");
   
           //Read updated config
           Console.WriteLine("AFTER[test]=" + config.AppSettings.Settings["test"].Value);
           Console.WriteLine("AFTER[testExternalOnly]=" + config.AppSettings.Settings["testExternalOnly"].Value);
   
           Console.WriteLine("AFTER CONFIG EXTERNAL FILE: " + System.IO.File.ReadAllText("App.Settings.config"));
   
           Console.WriteLine("AFTER CONFIG FILE: " + System.IO.File.ReadAllText(System.AppDomain.CurrentDomain.FriendlyName + ".config"));
   
   
           //Shut current config
           config = null;
   
           //Open config
           config = ConfigurationManager.OpenExeConfiguration(exepath);
           config.AppSettings.SectionInformation.ConfigSource = "App.Settings.config";
   
           Console.WriteLine("AFTER[test]=" + config.AppSettings.Settings["test"].Value);
           Console.WriteLine("AFTER[testExternalOnly]=" + config.AppSettings.Settings["testExternalOnly"].Value);
   
           Console.WriteLine("AFTER CONFIG EXTERNAL FILE: " + System.IO.File.ReadAllText("App.Settings.config"));
   
           Console.WriteLine("AFTER CONFIG FILE: " + System.IO.File.ReadAllText(System.AppDomain.CurrentDomain.FriendlyName + ".config"));
       }
       catch (Exception ex)
       {
           Console.WriteLine(ex.Message);
       }
       Console.WriteLine("press the ENTER key to end");
       Console.ReadLine();
   
   }
   

This will result in App.Settings.config file updated to be on the filesystem as: