private void DownloadFileAsyncBegin(string ticket, string fileId, string filePath)
{
_taskAsyncStarted = true;
var taskScheduler = TaskScheduler.FromCurrentSynchronizationContext();
var task = Task <bool> .Factory.StartNew(() => DokuFlexService.Download(ticket, fileId, filePath));
task.ContinueWith(t => DownloadFileAsyncEnd(t.Result), taskScheduler);
task.ContinueWith(t => TaskAsyncExceptionHandle(t.Exception),
new CancellationTokenSource().Token, TaskContinuationOptions.OnlyOnFaulted,
taskScheduler);
}