在C＃中从SQL Server流式传输VARBINARY数据

Question

I'm trying to serve image data stored in a VARBINARY(MAX) field in the database using ASP.Net. Right now, the code is filling a data table, then pulling the byte array out of the DataRow and pushing the byte array into the response. I'm wondering if there's a way to more-or-less stream the data from the SQL Server into the response without having to marshal around these huge byte arrays (since the images are large, they cause OutOfMemoryExceptions). Is there a class/mechanism for that?

The current code looks more or less like:

DataTable table = new DataTable();
SqlDataAdapter adapter = new SqlDataAdapter(commandText, connectionString);
adapter.Fill(table);
DataRow row = table.Rows[0];
byte[] imageData = row[0] as byte[];
if(imageData != null)
{
  Response.Clear();
  Response.BinaryWrite(imageData);
  Response.End();
}

Thanks in advance - any help is appreciated.

Answer 1

See Download and Upload Images from SQL Server for an article covering the topic, including efficient streaming semantics. You must use a SqlDataReader opened with CommandBehavior.SequentialAccess :

SequentialAccess Provides a way for the DataReader to handle rows that contain columns with large binary values. Rather than loading the entire row, SequentialAccess enables the DataReader to load data as a stream. You can then use the GetBytes or GetChars method to specify a byte location to start the read operation, and a limited buffer size for the data being returned.

The linked article provides full code for creating a Stream backed by an SqlDataReader, you can simply Stream.CopyTo (HttpResponse.OutputStream) , or use a byte[] chunked copy if you don't have .Net 4.0 yet.

This follow up article explains how to use a FILESTREAM column for efficient streaming of large VARBINARY data in and out of the database.

Answer 2

@Remus's answer above is out-of-date since .NET 4.5 introduced first-class SqlClient Streaming . It's no longer necessary to access the SqlDataReader's GetBytes() methods to get a stream from a SQL Query.

Now you simply call SqlDataReader.GetStream(int) to get a stream over a blob column.