A second operation was started on this context before a previous operation completed
解决异常情况,并发情况会导致异常,每次创建一个新上下文就解决了InvalidOperationException:Asecondoperationwasstartedonthiscontextbeforeapreviousoperationcompleted.Thisisusuallycausedbydifferentthreadsconcurrentlyusingthesameinstanceo