/// <summary>
 /// Gets a particular metadata tag's value.  Return null if the meta tag doesn't exist
 /// in this document.
 /// </summary>
 /// <param name="metaDataName">The name of the metadata element to retrieve</param>
 /// <returns>The value</returns>
 private string GetMetaDataValue(string metaDataName)
 {
     if (MetaTags.Contains(metaDataName))
     {
         return((string)MetaTags[metaDataName]);
     }
     else
     {
         return(null);
     }
 }