ASP.NET 6 - Asynchronous serialization of IAsyncEnumerable class property

Question

ASP.NET Core now supports async streaming from controller actions all the way down to the response JSON formatter. Returning an IAsyncEnumerable from an action no longer buffers the response content in memory before it gets sent. This helps reduce memory usage when returning large datasets that can be asynchronously enumerated.

Since DbSet implements IAsyncEnumerable, the following example is fully supported:

[ApiController]
public class Controller : ControllerBase
{
    private readonly DbContext _context;

    public Controller(DbContext context) => _context = context;

    [HttpGet]
    public IAsyncEnumerable<AspNetUser> GetData()
    {
        return _context.AspNetUsers.AsAsyncEnumerable();
    }
}

Seemingly this feature can only be utilized when returning the data source itself, however when the data source is wrapped, so that an additional metadata could be sent, streaming is not possible.

Basic API result:

public class DataSourceResult<T>
{
    public IEnumerable<T> Data { get; set; }
    public int PageIndex { get; set; }
    public int PageSize { get; set; }
    public int TotalCount { get; set; }
}

As many of my applications are running on low-memory environments, I'm looking for some suggestions on how to use async streaming in cases like this, where data itself is indirectly returned.

You could set some HTTP headers for the page size, index and count? — DavidG, Nov 10 '21 at 19:41

score 4 · Accepted Answer · answered Nov 10 '21 at 20:44

All you need to do is change your DataSourceResult<T> class to have a Data property type of IAsyncEnumerable<T>. The System.Text.Json serialiser recognises the type and streams it to the client. So:

public class DataSourceResult<T>
{
    public IAsyncEnumerable<T> Data { get; set; }
    public int PageIndex { get; set; }
    public int PageSize { get; set; }
    public int TotalCount { get; set; }
}

And your action would be something like this:

public DataSourceResult<AspNetUser> GetData()
{
    return new DataSourceResult<AspNetUser>
    {
        Data = _context.AspNetUsers.AsAsyncEnumerable(),
        PageIndex = 1,
        PageSize = 10,
        TotalCount = 12345
    };
}

ASP.NET 6 - Asynchronous serialization of IAsyncEnumerable class property

1 Answers1