How to return FileStreamResult with SqlDataReader - sql-server

Question

I have an ASP.NET Core project that downloads large files which are stored in SQL Server. It works fine for small files, but large files often time out as they are read into memory before getting downloaded.
So I am working to improve that.
Based on SQL Client streaming support examples I have updated the code to the following:
public async Task<FileStreamResult> DownloadFileAsync(int id)
{
ApplicationUser user = await _userManager.GetUserAsync(HttpContext.User);
var file = await this._attachmentRepository.GetFileAsync(id);
using (SqlConnection connection = new SqlConnection(this.ConnectionString))
{
await connection.OpenAsync();
using (SqlCommand command = new SqlCommand("SELECT [Content] FROM [Attachments] WHERE [AttachmentId] = #id", connection))
{
command.Parameters.AddWithValue("id", file.AttachmentId);
SqlDataReader reader = await command.ExecuteReaderAsync(CommandBehavior.SequentialAccess);
if (await reader.ReadAsync())
{
if (!(await reader.IsDBNullAsync(0)))
{
Stream stream = reader.GetStream(0);
var result = new FileStreamResult(stream, file.ContentType)
{
FileDownloadName = file.FileName
};
return result;
}
}
}
}
return null;
}
But when I test, it throws this exception:
Cannot access a disposed object. Object name: 'SqlSequentialStream'
Is there a way to fix this exception?

Answer 1

Your using statements are all triggering when you do your return, thus disposing your connection and command, but the whole point of this is to leave the stream copy to happen in the background after your function is done.
For this pattern you're going to have to remove the using calls and let garbage collection trigger when the stream copy is done. FileStreamResult should at the very least call Dispose on the stream you give it, which should un-root the command and connection to be later finalized and closed.

Answer 2

This is the working code, which is dramatically faster than without the streaming:
[HttpGet("download")]
public async Task<FileStreamResult> DownloadFileAsync(int id)
{
var connectionString = _configuration.GetConnectionString("DefaultConnection");
ApplicationUser user = await _userManager.GetUserAsync(HttpContext.User);
var fileInfo = await this._attachmentRepository.GetAttachmentInfoByIdAsync(id);
SqlConnection connection = new SqlConnection(connectionString);
await connection.OpenAsync();
SqlCommand command = new SqlCommand("SELECT [Content] FROM [Attachments] WHERE [AttachmentId]=#id", connection);
command.Parameters.AddWithValue("id", fileInfo.Id);
// The reader needs to be executed with the SequentialAccess behavior to enable network streaming
// Otherwise ReadAsync will buffer the entire BLOB into memory which can cause scalability issues or even OutOfMemoryExceptions
SqlDataReader reader = await command.ExecuteReaderAsync(CommandBehavior.SequentialAccess);
if (await reader.ReadAsync())
{
if (!(await reader.IsDBNullAsync(0)))
{
Stream stream = reader.GetStream(0);
var result = new FileStreamResult(stream, fileInfo.ContentType)
{
FileDownloadName = fileInfo.FileName
};
return result;
}
}
return null;
}