Asynchronously writes the XML declaration.
An XmlWriter method was called before a previous asynchronous operation finished. In this case, InvalidOperationException is thrown with the message “An asynchronous operation is already in progress.”
An XmlWriter asynchronous method was called without setting the Async flag to true. In this case, InvalidOperationException is thrown with the message “Set XmlWriterSettings.Async to true if you want to use Async Methods.”