c# - 为什么我不能将异步查询移动到方法中？

Question

下面的代码有效

[Route("case-studies/{slug}")]
public async Task<ActionResult> Details(string slug)
{
    var item = await Db.Pages.OfType<CaseStudy>()
             .WithSlug(slug)
             .FirstOrDefaultAsync();

    if (item == null)
    {
        return HttpNotFound();
    }

    var related = await Db.Pages.OfType<CaseStudy>()
           .Where(r => r.Client == item.Client && r.Id != item.Id)
           .Where(r => !r.IsArchived)
           .Include(r => r.Media)
           .Take(3)
           .Project()
           .To<RelatedPageModel>()
           .ToListAsync();

    var archived = await Db.Pages.OfType<CaseStudy>()
            .Where(r => r.Client == item.Client && r.Id != item.Id)
            .Where(r => r.IsArchived)
            .Take(3)
            .Project()
            .To<RelatedPageModel>()
            .ToListAsync();

    ViewData.Model = new DetailPageModel<CaseStudy>()
    {
        Item = item,
        RelatedItems = related,
        ArchivedItems = archived
    };

    return View();
}

但是，当我尝试按如下方式重构异步方法调用时

[Route("case-studies/{slug}")]
public async Task<ActionResult> Details(string slug)
{
    var item = await Db.Pages.OfType<CaseStudy>()
             .WithSlug(slug)
             .FirstOrDefaultAsync();

    if (item == null)
    {
        return HttpNotFound();
    }       

    var related = await GetRelatedCaseStudies(item, false);
    var archived = await GetRelatedCaseStudies(item, true);

    ViewData.Model = new DetailPageModel<CaseStudy>()
    {
        Item = item,
        RelatedItems = related,
        ArchivedItems = archived
    };

    return View();
}


private Task<List<RelatedPageModel>> GetRelatedCaseStudies(CaseStudy casestudy, bool archived)
{
    return Db.Pages.OfType<CaseStudy>()
            .Where(r => r.Client == casestudy.Client && r.Id != casestudy.Id)
            .Where(x => x.IsArchived == archived)
            .Include(r => r.Media)
            .Take(3)
            .Project().To<RelatedPageModel>()
            .ToListAsync();
}

它没有给我以下错误

A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe.

这是为什么？我怎样才能使它工作？

更新:

Db在base controller中声明如下

private WebSiteDb db;

protected WebSiteDb Db
{
    get
    {
        LazyInitializer.EnsureInitialized(ref db, () => new WebSiteDb());

        return db;
    }
}

WebSiteDb 如下扩展 DbContext

   [DbConfigurationType(typeof(DbConfig))]
    public class WebSiteDb : DbContext
    {
        static WebSiteDb() {
            Database.SetInitializer<WebSiteDb>(new WebSiteDbInitializer());
        }
        public IDbSet<Page> Pages { get; set; }
        public IDbSet<Media> Media { get; set; }
        ...some missing sets

        public WebSiteDb() : base("MyDatabaseName") { }
     }

如果我在方法内部 await，则错误会从方法内部抛出

WithSlug()如下

public static IQueryable<T> WithSlug<T>(this IQueryable<T> pages, string slug) where T : Page
        {
            return pages.Where(p => p.Slug == slug);
        }

Answer 1

用最新的 EF 6.1.0 Beta 试试你的代码.当前EF6 definition of thread safety有点模糊:

Thread Safety

While thread safety would make async more useful it is an orthogonal feature. It is unclear that we could ever implement support for it in the most general case, given that EF interacts with a graph composed of user code to maintain state and there aren't easy ways to ensure that this code is also thread safe.

For the moment, EF will detect if the developer attempts to execute two async operations at one time and throw.

看起来您的代码同时执行两个以上的异步操作，但在 ASP.NET 中，线程切换可能并且确实发生在 await 延续。理论上，EF6 应该仍然支持这种场景。但是，要消除因 ASP.NET 中缺少线程关联而导致的 EF6 错误的可能性，您可以尝试我的 ThreadWithAffinityContext 来自 the related question ，像这样:

public async Task<ActionResult> Details(string slug)
{
    Func<Task<ActionResult>> doAsync = async () =>
    {
        var item = await Db.Pages.OfType<CaseStudy>()
                 .WithSlug(slug)
                 .FirstOrDefaultAsync();

        if (item == null)
        {
            return HttpNotFound();
        }

        var related = await Db.Pages.OfType<CaseStudy>()
               .Where(r => r.Client == item.Client && r.Id != item.Id)
               .Where(r => !r.IsArchived)
               .Include(r => r.Media)
               .Take(3)
               .Project()
               .To<RelatedPageModel>()
               .ToListAsync();

        var archived = await Db.Pages.OfType<CaseStudy>()
                .Where(r => r.Client == item.Client && r.Id != item.Id)
                .Where(r => r.IsArchived)
                .Take(3)
                .Project()
                .To<RelatedPageModel>()
                .ToListAsync();

        ViewData.Model = new DetailPageModel<CaseStudy>()
        {
            Item = item,
            RelatedItems = related,
            ArchivedItems = archived
        };

        return View();
    };

    using (var staThread = new Noseratio.ThreadAffinity.ThreadWithAffinityContext(
        staThread: false, pumpMessages: false))
    {
        return await staThread.Run(() => doAsync(), CancellationToken.None);
    }
}

请注意，这不是生产解决方案，但它可能有助于发现 EF6 中的错误。 如果有错误，您可以考虑使用另一个辅助类，ThreadAffinityTaskScheduler ，直到该错误在未来的 EF 版本中得到修复。 ThreadAffinityTaskScheduler 运行一个 ThreadWithAffinityContext 线程池，因此应该比上面的代码更好地扩展。 linked question包含一个使用示例。