Change the Value in App.Config File Dynamically

How to update app settings key value pair dynamically on app.config file in c# winforms

Configuration configuration = ConfigurationManager.
OpenExeConfiguration(Assembly.GetExecutingAssembly().Location);
configuration.AppSettings.Settings["logPath"].Value = DateTime.Now.ToString("yyyy-MM-dd");
configuration.Save();
ConfigurationManager.RefreshSection("appSettings");

How to change values in config file dynamically in react?

You can accomplish it by using the state hook. Here's an example (You can see my code in action here: https://codesandbox.io/s/thirsty-keller-cjqb8):

// put this in config.js
const initialConfig = {
  darkMode: true
};

const App = () => {
  // dark mode is initally true, because we used the config value as inital value for darkMode
  const [darkMode, setDarkMode] = useState(initialConfig.darkMode);

  // change handler that is invoked when we change the value of the checkbox
  const changeMode = ({ currentTarget: { checked } }) => setDarkMode(checked);

  return (
    // use className based on the value of darkMode
    <div className={darkMode ? "darkMode" : ""}>
      <label>
        <input checked={darkMode} type="checkbox" onChange={changeMode} />
        use dark mode
      </label>
    </div>
  );
};

I used the config as an inital value. However, you need a state that stores the information which mode is currently active.

How to dynamically apply changes to app.config on program start?

You're not using the Config file appropriately. Try this:

<configuration>
...
    <appSettings>

        <add key="EnvUnderTest" value="settings_a.xml" />
    </appSettings>
...
</configuration>

This should work with the code you've provided. Obviously, any other key you wish to add should also be under <appSettings>.

Change default app.config at runtime

The hack in the linked question works if it is used before the configuration system is used the first time. After that, it doesn't work any more.

The reason:

There exists a class ClientConfigPaths that caches the paths. So, even after changing the path with SetData, it is not re-read, because there already exist cached values. The solution is to remove these, too:

using System;
using System.Configuration;
using System.Linq;
using System.Reflection;

public abstract class AppConfig : IDisposable
{
    public static AppConfig Change(string path)
    {
        return new ChangeAppConfig(path);
    }

    public abstract void Dispose();

    private class ChangeAppConfig : AppConfig
    {
        private readonly string oldConfig =
            AppDomain.CurrentDomain.GetData("APP_CONFIG_FILE").ToString();

        private bool disposedValue;

        public ChangeAppConfig(string path)
        {
            AppDomain.CurrentDomain.SetData("APP_CONFIG_FILE", path);
            ResetConfigMechanism();
        }

        public override void Dispose()
        {
            if (!disposedValue)
            {
                AppDomain.CurrentDomain.SetData("APP_CONFIG_FILE", oldConfig);
                ResetConfigMechanism();

                disposedValue = true;
            }
            GC.SuppressFinalize(this);
        }

        private static void ResetConfigMechanism()
        {
            typeof(ConfigurationManager)
                .GetField("s_initState", BindingFlags.NonPublic | 
                                         BindingFlags.Static)
                .SetValue(null, 0);

            typeof(ConfigurationManager)
                .GetField("s_configSystem", BindingFlags.NonPublic | 
                                            BindingFlags.Static)
                .SetValue(null, null);

            typeof(ConfigurationManager)
                .Assembly.GetTypes()
                .Where(x => x.FullName == 
                            "System.Configuration.ClientConfigPaths")
                .First()
                .GetField("s_current", BindingFlags.NonPublic | 
                                       BindingFlags.Static)
                .SetValue(null, null);
        }
    }
}

Usage is like this:

// the default app.config is used.
using(AppConfig.Change(tempFileName))
{
    // the app.config in tempFileName is used
}
// the default app.config is used.

If you want to change the used app.config for the whole runtime of your application, simply put AppConfig.Change(tempFileName) without the using somewhere at the start of your application.

App.Config change value

You cannot use AppSettings static object for this. Try this

string appPath = System.IO.Path.GetDirectoryName(Reflection.Assembly.GetExecutingAssembly().Location);          
string configFile = System.IO.Path.Combine(appPath, "App.config");
ExeConfigurationFileMap configFileMap = new ExeConfigurationFileMap();         
configFileMap.ExeConfigFilename = configFile;          
System.Configuration.Configuration config = ConfigurationManager.OpenMappedExeConfiguration(configFileMap, ConfigurationUserLevel.None);

config.AppSettings.Settings["YourThing"].Value = "New Value"; 
config.Save(); 

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