title: C# throws a ConfigurationErrorsException upon loading App.config
body: <p>The only solutions I've been able to find on this subject is to add the keys inside the appSettings tag, but as you can see I have already done that.</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8" ?&gt;
  &lt;configuration&gt;
    &lt;appSettings&gt;
      &lt;add key="customFilename" value="55"/&gt;
    &lt;/appSettings&gt;
  &lt;/configuration&gt;
  </code></pre>
  
  <p>The code loading the value throws an error here</p>
  
  <pre><code>private void reloadConfigFile()
  {
      MessageBox.Show(ConfigurationManager.AppSettings["customFilename"]);
  }
  </code></pre>
  
  <p>I've even copied and pasted someone else's App.config file, which was working for them, and it still failed to compile. What did I miss?</p>