Modify App.Config At Run Time

ModifyAppConfig.cs

using System;
using System.Configuration;
using System.Diagnostics;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            var configFile = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);

            configFile.AppSettings.Settings.Add("key1", "value1");
            configFile.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection(configFile.AppSettings.SectionInformation.Name);

            Debug.Assert(ConfigurationManager.AppSettings["key1"] == "value1");

            configFile.AppSettings.Settings["key1"].Value = "value2";
            configFile.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection(configFile.AppSettings.SectionInformation.Name);

            Debug.Assert(ConfigurationManager.AppSettings["key1"] == "value2");

            configFile.ConnectionStrings.ConnectionStrings.Add(new ConnectionStringSettings("Conn1", "ConnectionString1"));
            configFile.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection(configFile.ConnectionStrings.SectionInformation.Name);

            Debug.Assert(ConfigurationManager.ConnectionStrings["Conn1"].ConnectionString == "ConnectionString1");

            configFile.ConnectionStrings.ConnectionStrings["Conn1"].ConnectionString = "ConnectionString2";
            configFile.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection(configFile.ConnectionStrings.SectionInformation.Name);

            Debug.Assert(ConfigurationManager.ConnectionStrings["Conn1"].ConnectionString == "ConnectionString2");

            Console.ReadLine();
        }
    }
}