在ASP.NET Core中流式处理IEnumerable

Question

I am trying to implement a controller in ASP.NET core that returns a potentially huge amount of data queried from an SQL database. To achieve this I am executing the query via Dapper's Query() method with the buffered: false parameter. This means the SqlConnection object must be disposed after the query results have been enumerated.

There does not seem to be an easy way to achieve this. I have got it working by writing a class that wraps the IEnumerable, and reference counts any Enumerators it is asked to create. When all Enumerators are disposed it disposes of the database connection.

This seems to work but I'm worried there are cases where the database connection wouldn't be disposed (eg if the request is cancelled before it starts enumerating the query results). Is there a better approach, short of writing to a temporary file and streaming that to the client?

Answer 1

This is what I came up with in the end. The lifetime of the connection is owned by the StreamedDatabaseObjectResult which is returned in place of the usual ObjectResult. The repository class returns the connection to dispose along with the results. This seems to cover all cases and plays nicely with async methods.

public class StreamedDatabaseObjectResult : ActionResult
{
    private readonly Func<Task<DisposableEnumerable>> _getValuesFunc;

    public StreamedDatabaseObjectResult(Func<Task<DisposableEnumerable>> getValuesFunc)
    {
        _getValuesFunc = getValuesFunc;
    }

    public override async Task ExecuteResultAsync(ActionContext context)
    {
        using (var de = await _getValuesFunc())
        {
            var objectResult = new ObjectResult(de.Values);
            await objectResult.ExecuteResultAsync(context);
        }
    }
}

public class DisposableEnumerable : IDisposable
{
    public IEnumerable Values { get; }
    private readonly IDisposable _disposable;

    public DisposableEnumerable(IDisposable disposable, IEnumerable values)
    {
        Values = values;
        _disposable = disposable;
    }

    /// <inheritdoc />
    public void Dispose()
    {
        _disposable.Dispose();
    }
}

[HttpGet]
[ProducesResponseType(200)]
public ActionResult<IEnumerable<MyClass>> GetThings(CancellationToken cancellationToken)
{
    return new StreamedDatabaseObjectResult(() => _repo.GetThings(cancellationToken));
}