Streaming Extensions for LINQ to DB with SQL Server

The goal is to upload and download large files with minimal memory allocation.

By default, SQL Server does not support true streaming for uploads—unless you're on Windows and have the FILESTREAM feature enabled in your database.
Additionally, LINQ to DB does not natively support streaming or range-based downloads from SQL Server.

1. Upload

When uploading a large file to the database, the entire file is typically loaded into memory before being sent to SQL Server.

The only way to avoid this is to write the data in chunks using SQL Server’s .WRITE() method (see SQL Server documentation).

However, this approach requires multiple separate SQL commands—the number depending on the file size and chunk size.
Therefore, it’s only worthwhile for files larger than 20–50 MB, where memory usage would otherwise become a concern.

2. Download

When downloading binary data via LINQ to DB, there is no built-in way to stream directly from SQL Server (e.g., to HttpResponse.Body without buffering the entire file in memory).

This extension adds true streaming support for downloads, enabling efficient, low-memory file delivery.

Examples

Sample table:

CREATE TABLE [dbo].[Files](
	[Id] [int] IDENTITY(1,1) NOT NULL,
	[FileData] [varbinary](max) NULL,
)

DbContext:

public class DbContext : DataContext
{
	public DbContext(string connectionString) : base(ProviderName.SqlServer, connectionString)
	{
		CommandTimeout = 600;
	}
}

FileService:

private DbContext db;

public async Task<int> Upload(Stream? fileStream, string fileName, CancellationToken cancellationToken)
{
	using (var t = await db.BeginTransactionAsync()) // we need a transaction here to avoid partially loaded data in case of errors
	{
		var file = new Model.File { FileData = new byte[0] };
		var id = db.InsertWithInt32Identity(file);

		var chunksCount = await db.StreamUpload<Model.File>(id, x => x.FileData, fileStream, cancellationToken: cancellationToken);
		await t.CommitTransactionAsync(cancellationToken);

		return id;
	}
}

public async Task CopyToStream(long id, Stream destStream, CancellationToken cancellationToken)
{
	await db.CopyToStream<Model.File>(id, x => x.FileData, destStream, cancellationToken: cancellationToken);
}

Additional settings available:

StreamingSettings.UploadBufferSize = 11_000_000; // in bytes
StreamingSettings.DownloadBufferSize = 81920; // in bytes
StreamingSettings.PassThroughOperationCancelledExceptions = false;
StreamingSettings.UseArrayPool = true;

Example of file upload (~ 890 MB) with buffer = 11 MB

File length: 890536451 bytes
Content-Type: application/zip
1: Bytes read: 11000000,
2: Bytes read: 11000000,
3: Bytes read: 11000000,
...
81: Bytes read: 10536451,
Total: 890536451
File successfully uploaded for: 00:00:14.7621856