在DbContext中使用语句ToListAsync():"ObjectContext disposed",如何处理？

Question

我正在尝试使用.ToListAsync()Inside a DbContext的using语句获取linq查询结果,代码:

private async Task<List<String>> GetEntiteesAsync()
{
     Task<List<String>> returnValue;

     using (var entities = new REPORTEntities())
     {
          returnValue = (from user in entities.USERs
                         group user by user.entite into g
                               select g.Key).ToListAsync();
     }

     return await returnValue;
}

在运行时,我得到"已经处理了ObjectContext实例,不能再用于需要连接的操作." 如图所示 :

我认为这是由于在returnValue对象仍然像List一样异步接收对象的情况下处理Context这一事实引起的,是否有一种解决方法可以在保留using语句时避免此错误,或者我应该这样做:

private async Task<List<String>> GetEntiteesAsync()
{
     Task<List<String>> returnValue;

     var entities = new REPORTEntities()

     returnValue = (from user in entities.USERs
                    group user by user.entite into g
                    select g.Key).ToListAsync();

     return await returnValue;
}

Answer 1

您将using在ToListAsync操作完成之前离开作用域,因为您没有等待异步任务,这意味着entities处理太快(因此对象处置异常).

您应该只在范围内返回结果,并且async-await机制将确保Dispose在操作异步完成后调用:

private async Task<List<String>> GetEntiteesAsync()
{
     using (var entities = new REPORTEntities())
     {
          return await (from user in entities.USERs
                         group user by user.entite into g
                               select g.Key).ToListAsync();
     }
}