Is it necessary to set the Content-Length in my response header?

Question

I'm reviewing some legacy code and I've found a bug that causes the response to sit indefinitely.

Here's the basic idea:

Response.Content-Type = "application/octet-stream" Response.AddHeader("Content-Disposition", "attachment; filename" & someFileName) Response.AddHeader("Content-Length", someStoredLength) Response.BinaryWrite(someByteArray) Response.Flush() Response.End()

The problem is that someStoredLength is much larger than the actual size of someByteArray, so the client just sits there waiting for the file download while the browser just spins.

I'm contemplating just removing the AddHeader that specifies the content length, because when I do that everything seems to work fine, but I'm worried that I'm not understanding something.

Is it ok for me to remove this AddHeader or should I figure out a better way to deal with this problem?

What language is this? What class is the Response object in the above code? — noctonura
– noctonura, Commented Sep 11, 2009 at 18:43
@RichAmberale: That's not really relevant to the question. The issue occurs at the browser due to the HTTP headers. — Ben S
– Ben S, Commented Sep 11, 2009 at 18:47
The code is in VB.NET but I might find this in other places where the legacy is done in ASP classic — Joseph
– Joseph, Commented Sep 11, 2009 at 18:48

Ben S · Accepted Answer · 2009-09-11 18:44:59Z

12

Your application SHOULD (scroll down to Content-Length) define it, however, it's not strictly required.

Here's a decent discussion of possible options.

answered Sep 11, 2009 at 18:44

Ben S

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2 Comments

lxgr Over a year ago

The proposed solution in the linked article ("just set the length to some arbitrary value which will be likely too big") seems like a REALLY bad idea. Even if it doesn't break anything with current user agents, it undermines the whole concept of the "Content-length" header, and might break uncommon, but completely standards-compliant HTTP client libraries. If the size of the file is not known in advance, chunked transfer encoding should be used in all cases (and must be used if the connection is to be reused (keep-alive)).

paulkmoore Over a year ago

Direct link to Content-Length, see also HttpBis

Stephen · Accepted Answer · 2015-06-24 20:28:45Z

10

Change the Content-Length line to the following:

Response.AddHeader("Content-Length", someByteArray.Length.ToString())

edited Jun 24, 2015 at 20:28

answered Sep 11, 2009 at 18:48

Stephen

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2 Comments

Joseph Over a year ago

I was thinking of doing that too. I was wondering if that would be a good alternative. If I have a byte array will the Length property always give me the correct size?

Stephen Over a year ago

Yes. The content-length header indicates the number of bytes in the content. Your content is an array of bytes, so you're good.

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