How to write to the app.config file?

I've got a windows forms application and I would like to save a string in the app.config file.

I've googled this and can't find any example code that works.

All I want to do is read a string in when I load my form, and save it after a particular action. The idea is just to persist a setting on the form so the user doesn't have to keep re-entering it each time the form opens.

If I set the value manually I find the string can be read with no problems, but trying to write it just doesn't seem to save it.

Here is my settings file

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="LastSearchPath" value=""/>
  </appSettings>

Here is my current code:

            System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);            
            config.AppSettings.Settings["LastSearchPath"].Value = txtPath.Text;
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("appSettings");