How to return a List of objects from an async task

Question

I'm trying to implement .NET 4.5 async/await and am running into a few issues.

I have this code:

private async void GetPages()
{
    PageList = await _dataService.GetPageList(JobTypeId);
}

IDataService looks like this

public interface IDataService
{
    Task<List<PageDto>> GetPageList(int jobTypeId);
}

and DataService.GetPageList like this:

public Task<List<PageDto>> GetPageList(int jobTypeId)
{
    var retval = new List<PageDto>();

    try
    {
        retval =
            (from p in _connection.Table<PageDto>()
                where p.JobTypeId == jobTypeId
                select p).ToList();
    }

    catch (Exception ex)
    {
        Logger.Exception(ex);
    }

    return retval;
}

I somehow need to wrap the result in a task but I can't figure out how.

Any ideas?

UPDATE

I can't use ToListAsync due to limitations in the SQLLite library I'm using but how about something like this?

public async Task<List<PageDto>> GetPageListAsync(int jobTypeId)
{
    var retval = new List<PageDto>();

    try
    {
        await Task.Run(() =>
        {
            retval =
                (from jtp in _connection.Table<JobTypePage>()
                 join p in _connection.Table<PageDto>() on jtp.PageId equals p.PageId
                 where jtp.JobTypeId == jobTypeId
                 orderby jtp.JobTypePageId
                 select p).ToList();
        });

    }

    catch (Exception ex)
    {
        Logger.Exception(ex);
    }

    return retval;
}

Stephen Cleary · Accepted Answer · 2014-11-11 22:05:20Z

5

First, you should be using asynchronous methods from the inside-out (not "imposing" async from the outside-in). In this case, I assume you're using EF6, so you could do:

retval = await
    (from p in _connection.Table<PageDto>()
        where p.JobTypeId == jobTypeId
        select p).ToListAsync();

And then you can just mark your method async:

public async Task<List<PageDto>> GetPageListAsync(int jobTypeId)

As I side note, I added the Async suffix to follow the Task-based Asynchronous Pattern.

For more information, check out my async intro.

answered Nov 11, 2014 at 22:05

Stephen Cleary

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8 Comments

Sriram Sakthivel Over a year ago

Stephen, Any reason for not returning task returned from ToListAsync and awaiting it?

Stephen Cleary Over a year ago

It changes the error handling in this example. The error handling strategy the op is using is not one I would choose myself, but if the rest of the code assumes errors are logged and then ignored by the data service, then it's important for consistency.

Steve Chadbourne Over a year ago

It's SQLLite which does not have a ToListAsync method. I'm attempting to upgrade an old app that used backgroundworker.

Steve Chadbourne Over a year ago

If I can't use async from the inside out - due to the SQlLite library I'm using not supporting async - how would I then impose async from the outside in? Or is there another way?

Stephen Cleary Over a year ago

Newer versions of SQLite (at least SQLite.NET) do support async. So check for a new version first, but if it still doesn't support it, then the proper solution is to return a plain List<T> from IDataService.GetPageList and not use async at all there. Then use Task.Run when you call it from your UI layer: PageList = await Task.Run(() => _dataService.GetPageList(JobTypeId));

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