Entity framework 4 and sql filestream to read a file from db

Question

I'm using ASP.NET 4, EF 4 and FILESTREAM in SQL 2008 to add/read files to the DB. I'm able to upload files just fine, but I'm not able to retrieve the files the way I want to. Here's what I'm doing -

I have a grid which displays a list of files. Each line item is a file with a CommandName and CommandArgument set.
The user clicks on any one of these files and I capture the event in the RowCommand event. At this point, I get the ID of the file and retrieve the byte[] from the DB. How do I display a standard download box for the user to Save or Cancel the download?

I can write the stream to a temp file (using System.IO.File.WriteAllBytes) and then do a Server.Transfer to the temp file but I think it's an unnecessary step. Is there a way, I can read the bytes directly to memory, then set the ContentType/MimeType and allow the user to save the file?

I understand that accessing the FILESTREAM file using T-SQL is slower than accessing the same using WIN32 API's (using a combination of the new SystemFile.PathName() & Impersonation) but EF4 makes working with SQL faster so I'm going that route.

Here's the code -

var file = db.Storage.Single(f => f.ID.Equals(fileID)); // fileID is set in CommandArgument
// file.Contents has the byte [].
// display a  standard file download box.

Thanks for any help!

Zachary Yates · Accepted Answer · 2010-09-15 20:54:06Z

1

You probably want to create an .ashx hanlder (assuming you are using asp) and to a Server.Transfer() or Server.Redirect() to that URL.

Alternatively, you could change the GridView to display a hyperlink to the handler URL as well.

Right click in Solution Explorer > New Item > ASP.Net Hanlder.

Code for handler:

public class DownloadFile : IHttpHandler { // add IRequiresSessionState if needed 
  public bool IsReusable { get { return true; } }

  public void ProcessRequest(HttpContext context) {
    var fileID = context.Request.QueryString["fileID"]; // assuming fileID is a string
    var file = db.Storage.Single(f => f.ID.Equals(fileID)); 
    // set the content type
    context.Response.OutputStream.Write(file.Contents, 0, file.Contents.Length);
  }
}

You probably want to add some error checking as well.

Your URL would be something like this: /DownloadFile.ashx?fileID=somefileid

edited Sep 15, 2010 at 20:54

answered Sep 15, 2010 at 20:46

Zachary Yates

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7 Comments

tempid Over a year ago

Yes, I'm using ASP.NET (updated the question with this info). What URL are you talking about? All I have is bytes. Do you want me to write the bytes to a file and do a redirect to that file? If so, I'm trying to avoid that route as I think it may be necessary.

Zachary Yates Over a year ago

No need to write it to a file, you can write it directly to the OutputStream. I added code to better illustrate the point.

tempid Over a year ago

I think I see what you're saying. The link will look something like /GetFile.ashx?id={ID}, and the handler will read the bytes from the DB and write the bytes to the response stream directly without saving to disk?

tempid Over a year ago

Thanks for the code sample, I'll give it a shot and report back.

Zachary Yates Over a year ago

Exactly. Should also make your grid a little simpler.

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