Async Controller not returning anything to the view when querying multiple datatable in parallel

问题

Why is this Task not returning anything to the view? If I convert it to synchronous then it works, or if I step through the async task in visual studio, it works also.

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);
}

回答1:

Entity Framework can not handle multiple concurrent requests using a single DbContext. You need to await each list before you move on to the next one or use 3 separate DbContexts.

Here is how to do it just awaiting each request individually.

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);
}

With separate contexts.

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);
    }
}

Note using the separate context method you are disposing of them so if you have navigation properties that are lazy loaded you use then those navigation properties will nolonger work. I would recommend using the first method of using a single context and awaiting each list so you keep the DbContext alive for future requests using the lazy loaded navigation properties.

来源：https://stackoverflow.com/questions/30446100/async-controller-not-returning-anything-to-the-view-when-querying-multiple-datat