title: Write appSettings in external file
body: <p>I have a config file app.exe.config and appSettings section has something like this:</p>
  
  <pre><code>&lt;configuration&gt;
      &lt;appSettings configSource="app.file.config" /&gt;
  &lt;/configuration&gt;
  </code></pre>
  
  <p>app.file.config file has something like this:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8" ?&gt;
  &lt;appSettings&gt;
    &lt;add key="var1" value="value 1" /&gt;
    &lt;add key="var2" value="value 2" /&gt;
    &lt;add key="var3" value="value 3" /&gt;
  &lt;/appSettings&gt;
  </code></pre>
  
  <p>I need to edit var1, var2 and var3 at runtime and I have code like this:</p>
  
  <pre><code>Configuration config = ConfigurationManager.OpenExeConfiguration("...path\app.exe);
  
  config.AppSettings.SectionInformation.ConfigSource = "app.file.config";
  
  config.AppSettings.Settings["var1"].Value = "value 11";
  config.AppSettings.Settings["var2"].Value = "value 22";
  config.AppSettings.Settings["var3"].Value = "value 33";
  config.Save(ConfigurationSaveMode.Modified);
  
  ConfigurationManager.RefreshSection("appSettings");
  </code></pre>
  
  <p>When I run config.Save.... the file app.file.config has a appSettings node with an attribute "file". This attribute has the value to app.file.config</p>
  
  <pre><code>&lt;appSettings file="app.file.config"&gt;
  &lt;add key="var1" value="value 1" /&gt;
    &lt;add key="var2" value="value 2" /&gt;
    &lt;add key="var3" value="value 3" /&gt;
  &lt;/appSettings&gt;
  </code></pre>
  
  <p>Now, if I try to load the config file, I have an exception with message "Unrecognized attribute 'file'. Note that attribute names are case-sensitive." in app.file.config.</p>
  
  <p>If I delete the file attribute manually, the configuration file is loaded properly.</p>
  
  <p>Any ideas?</p>
  
  <p>How can avoid to write file attribute when I save config files.</p>
  
  <p>Thanks</p>