c# - 并行查询多个数据表时，异步 Controller 不会向 View 返回任何内容

Question

为什么此任务没有向 View 返回任何内容？如果我将其转换为同步，那么它就可以工作，或者如果我在 Visual Studio 中单步执行异步任务，它也可以工作。

public async Task<ActionResult> Load()
{
    var housing = _db.Housing.ToListAsync();
    var school = _db.Schools.ToListAsync();
    var projects = _db.Projects.ToListAsync();

    await Task.WhenAll(housing, school, projects);

    var vm = new ItemViewModel
    {
        HousingTotals = await housing,
        SchoolTotals = await school,
        ProjectTotals = await projects
    };

    return PartialView(vm);
}

Answer 1

Entity Framework 无法使用单个 DbContext 处理多个并发请求。您需要等待每个列表，然后才能转到下一个列表或使用 3 个单独的 DbContext。

以下是如何单独等待每个请求。

public async Task<ActionResult> Load()
{
    var housing = await _db.Housing.ToListAsync();
    var school = await _db.Schools.ToListAsync();
    var projects = await _db.Projects.ToListAsync();


    var vm = new ItemViewModel
    {
        HousingTotals = housing,
        SchoolTotals = school,
        ProjectTotals = projects
    };

    return PartialView(vm);
}

具有单独的上下文。

private static YourContext GetContext()
{
    return new YourContext();//Change as needed.
}


public async Task<ActionResult> Load()
{
    using(var db1 = GetContext())
    using(var db2 = GetContext())
    using(var db3 = GetContext())
    {
        var housing = db1.Housing.ToListAsync();
        var school = db2.Schools.ToListAsync();
        var projects = db3.Projects.ToListAsync();

        await Task.WhenAll(housing, school, projects);

        var vm = new ItemViewModel
        {
            HousingTotals = await housing,
            SchoolTotals = await school,
            ProjectTotals = await projects
        };

        return PartialView(vm);
    }
}

请注意，您使用单独的上下文方法来处理它们，因此如果您使用延迟加载的导航属性，那么这些导航属性将不再起作用。我建议使用第一种方法，即使用单个上下文并等待每个列表，以便使用延迟加载导航属性使 DbContext 保持事件状态以供将来的请求。