title: app.config are not saving the values
body: <p>My <code>App.Config</code> is something like:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8" ?&gt;
  &lt;configuration&gt;
   &lt;appSettings&gt;
    &lt;add key="foo" value=""/&gt;
  &lt;/appSettings&gt;
  &lt;/configuration&gt;
  </code></pre>
  
  <p>I try to save the <code>foo</code> value using the following method:</p>
  
  <pre><code>private void SaveValue(string value) {
      var config =
          ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
      config.AppSettings.Settings.Add("foo", value);
      config.Save(ConfigurationSaveMode.Modified); 
  }
  </code></pre>
  
  <p>but this not change the value of it. and I don't get a exception.
   how to fix this? thanks in advance!</p>