using System;
using System.Configuration;

namespace DemoAppSettingsProblem
{
    class Program
    {
        static void Main(string[] args)
        {
            Configuration userConfig =
                ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.PerUserRoamingAndLocal);
            if ( userConfig.AppSettings.Settings["foo"] != null )
                userConfig.AppSettings.Settings.Remove("foo");

            userConfig.AppSettings.Settings.Add("foo", "The string is foo.");

            userConfig.Save(ConfigurationSaveMode.Modified);    // exception!
        }
    }
}