c# – 无法强制转换为我的自定义ConfigurationSection

var dllPath =  System.IO.Path.GetDirectoryName(System.Reflection.Assembly.
    GetExecutingAssembly().GetName().CodeBase);
dllPath = dllPath.Replace("file:",string.Empty);
var configPath = string.Format(@"{0}..contentFolders.config",dllPath);
var fileMap = new ExeConfigurationFileMap() {ExeConfigFilename = configPath};
var config = ConfigurationManager.OpenMappedExeConfiguration(fileMap,ConfigurationUserLevel.None);
var contentFolderConfig = 
    (ContentFolderSettings)config.GetSection("contentFolderConfiguration");

我有在CorpCommon项目中定义的ContentFolderSettings,它继承自ConfigurationSection.以下是contentFolders.config的内容：

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <section name="contentFolderConfiguration"
    type="Corp.Common.ContentFolderSettings,Corp.Common"
    requirePermission="false"/>
  <contentFolderConfiguration>
    <contentFolders>
      <contentFolder key="ImagesFolder" path="contentimages"/>
      <contentFolder key="CssFolder" path="contentcss"/>
      ...
    </contentFolders>
  </contentFolderConfiguration>
</configuration>

但调用config.GetSection()的行正在抛出InvalidCastException：

Unable to cast object of type 'System.Configuration.DefaultSection' to type 
'Corp.Common.ContentFolderSettings'.