Is Response.End() considered harmful?

Asked 6/7, 2009 at 15:58 Answered 27/3, 2019 at 18:9

203

This KB Article says that ASP.NET's Response.End() aborts a thread.

Reflector shows that it looks like this:

public void End()
{
    if (this._context.IsInCancellablePeriod)
    {
        InternalSecurityPermissions.ControlThread.Assert();
        Thread.CurrentThread.Abort(new HttpApplication.CancelModuleException(false));
    }
    else if (!this._flushing)
    {
        this.Flush();
        this._ended = true;
        if (this._context.ApplicationInstance != null)
        {
            this._context.ApplicationInstance.CompleteRequest();
        }
    }
}

This seems pretty harsh to me. As the KB article says, any code in the app following Response.End() will not be executed, and that violates the principle of least astonishment. It's almost like Application.Exit() in a WinForms app. The thread abort exception caused by Response.End() is not catchable, so surrounding the code in a try...finally won't satisfy.

It makes me wonder if I should always avoid Response.End().

Can anyone suggest, when should I use Response.End(), when Response.Close() and when HttpContext.Current.ApplicationInstance.CompleteRequest()?

ref: Rick Strahl's blog entry.

Based on the input I've received, my answer is, Yes, Response.End is harmful, but it is useful in some limited cases.

use Response.End() as an uncatchable throw, to immediately terminate the HttpResponse in exceptional conditions. Can be useful during debugging also. Avoid Response.End() to complete routine responses.
use Response.Close() to immediately close the connection with the client. Per this MSDN blog post, this method is not intended for normal HTTP request processing. It’s highly unlikely that you would have a good reason to call this method.
use CompleteRequest() to end a normal request. CompleteRequest causes the ASP.NET pipeline to jump ahead to the EndRequest event, after the current HttpApplication event completes. So if you call CompleteRequest, then write something more to the response, the write will be sent to the client.

Edit - 13 April 2011

Further clarity is available here:

Down answered 6/7, 2009 at 15:58 Comment(2)

no idea what's changed since this answer, but I am catching the Response.End ThreadAbortException just fine. – Tertius 9/10, 2014 at 16:16

Bear in mind also that Response.Redirect and Server.Transfer both call Response.End and should also be avoided. – Eulogist 25/11, 2014 at 11:55

If you had employed an exception logger on your app, it will be watered down with the ThreadAbortExceptions from these benign Response.End() calls. I think this is Microsoft's way of saying "Knock it off!".

I would only use Response.End() if there was some exceptional condition and no other action was possible. Maybe then, logging this exception might actually indicate a warning.

Indianapolis answered 6/7, 2009 at 18:6 Comment(0)

109

TL;DR

Initially I had recommended that you should simply replace all of your calls to [Response.End] with [...] CompleteRequest() calls, but if you want to avoid postback processing and html rendering you'll need to add [...] overrides as well.

Jon Reid, "Final Analysis"

Per MSDN, Jon Reid, and Alain Renon:

ASP.NET Performance - Exception Management - Write Code That Avoids Exceptions

The Server.Transfer, Response.Redirect, Response.End methods all raise exceptions. Each of these methods internally call Response.End. The call to Response.End, in turn, causes a ThreadAbortException exception.

ThreadAbortException Solution

HttpApplication.CompleteRequest() sets a variable that causes the thread to skip past most of the events in the HttpApplication event pipeline [--] not the Page event chain but the Application event chain.

...

create a class level variable that flags if the Page should terminate and then check the variable prior to processing your events or rendering your page. [...] I would recommend just overriding the RaisePostBackEvent and Render methods

Response.End and Response.Close are not used in normal request processing when performance is important. Response.End is a convenient, heavy-handed means of terminating request processing with an associated performance penalty. Response.Close is for immediate termination of the HTTP response at the IIS/socket level and causes issues with things like KeepAlive.

The recommended method of ending an ASP.NET request is HttpApplication.CompleteRequest. Keep in mind that ASP.NET rendering will have to be skipped manually since HttpApplication.CompleteRequest skips the rest of the IIS/ASP.NET application pipeline, not the ASP.NET Page pipeline (which is one stage in the app pipeline).

Code

Reference

HttpApplication.CompleteRequest

Causes ASP.NET to bypass all events and filtering in the HTTP pipeline chain of execution and directly execute the EndRequest event.

Response.End

This method is provided only for compatibility with ASP—that is, for compatibility with COM-based Web-programming technology that preceded ASP.NET.preceded ASP.NET. [Emphasis added]

Response.Close

This method terminates the connection to the client in an abrupt manner and is not intended for normal HTTP request processing. [Emphasis added]

Umbilication answered 12/10, 2010 at 17:3 Comment(6)

>Keep in mind that ASP.NET rendering will have to be skipped manually since HttpApplication.CompleteRequest skips the rest of the IIS/ASP.NET application pipeline, not the ASP.NET Page pipeline (which is one stage in the app pipeline). And how do you accomplish this? – Proselyte 18/10, 2012 at 16:53

See the link to the code where Jon Reid demonstrated how to set a flag and override the Page's RaisePostBackEvent and Render methods to skip the normal implementation when desired. (You'd probably do this in a base class all your app's pages should inherit from.) web.archive.org/web/20101224113858/http://www.c6software.com/… – Umbilication 19/10, 2012 at 18:35

Just to restate: HttpApplication.CompleteRequest does not terminate the response as Response.End does. – Anoa 29/11, 2013 at 20:38

HttpApplication.CompleteRequest also doesn't stop code flow, so subsequent lines keep running. That may not affect what the browser sees, but if those lines do any other processing, it can be really confusing. – Ernestoernestus 12/9, 2014 at 10:12

I cannot think but that Web Forms is broken by design. What is more performance degrading, calling Response.End() or letting the page load everything and then suppress the response? I cannot see where Response.End() is "more" harmful here. Moreover, Microsoft treats `ThreadAbortedException' as a normal event as evident from this code: referencesource.microsoft.com/#System.Web/UI/Page.cs,4875 One thing that is against Response.End() is that it might fail aborting the response, which might result occasionally in the response being displayed. – Delude 31/12, 2015 at 9:37

If you happen to do this work in Application_BeginRequest(), you don't need to worry about preventing the rest of the ASP.Net application pipeline, because MVC/Web Forms/etc is not going to trigger - all the events that would spin it up are skipped as the IIS pipeline now leaps ahead to the EndRequest() event. – Otoscope 21/6, 2018 at 14:22

106

This question appears near the top of all google searches for information on response.end so for other searches like myself who wish to post CSV/XML/PDF etc in response to an event without rendering the entire ASPX page, this is how I do it. (overriding the render methods is overly complex for such a simple task IMO)

// Add headers for a csv file or whatever
Response.ContentType = "text/csv"
Response.AddHeader("Content-Disposition", "attachment;filename=report.csv")
Response.AddHeader("Pragma", "no-cache")
Response.AddHeader("Cache-Control", "no-cache")

// Write the data as binary from a unicode string
Dim buffer As Byte()
buffer = System.Text.Encoding.Unicode.GetBytes(csv)
Response.BinaryWrite(buffer)

// Sends the response buffer
Response.Flush()

// Prevents any other content from being sent to the browser
Response.SuppressContent = True

// Directs the thread to finish, bypassing additional processing
HttpContext.Current.ApplicationInstance.CompleteRequest()

Trover answered 16/7, 2012 at 13:28 Comment(7)

You shouldn't be using an APSX page to do this. It's a lot of wasted effort. You are supposed to use an ASMX or Web Service, anything but an ASPX page. – Harriettharrietta 19/3, 2013 at 10:52

This seems to be the answer with the easiest implementation. The key is Response.SuppressContent = True. – Jehu 28/3, 2013 at 22:37

@Harriettharrietta - It's not always easy/best/advisable to have a separate page for different representation of the same resource. Think about REST, etc. If the client indicates they want csv, xml via a header or param, this method would certainly be the best, while still providing html support through asp.net's normal rendering facilities. – Jehu 28/3, 2013 at 22:39

This didn't work for me. I had a page that worked with Response.End(), but using all kinds of combinations of Response.Close(), Response.Flush(). HttpContext.Current.ApplicationInstance.CompleteRequest() and various other things did not work if I had a GzipStream filter on the Response. What appeared to be happening was that the page was still being output along with my file. I finally overrode the Render() function (to be blank) and that solved it for me. – Zumwalt 7/8, 2014 at 23:26

@Jay why is the Response.SuppressContent = True call necessary in this code? Doesn't the CompleteRequest() call finish everything properly? Could the Response.SuppressContent = True prevent the previous Response.Flush() line from completing its work? Thanks in advance... – Fourpenny 22/12, 2016 at 10:34

CompleteRequest skips parts of the application pipeline but will still run through the remainder of the page rendering process, its not an immediate stop like response.end, its more graceful. There are more in depth explanations as to why in other answers on this page. – Trover 28/12, 2016 at 13:14

Response.SuppressContent = True does not play nice with GZipStream filter, but you could call Reponse.Output.Close() instead. – Kathe 6/6, 2023 at 22:6

I would only use Response.End() if there was some exceptional condition and no other action was possible. Maybe then, logging this exception might actually indicate a warning.

Indianapolis answered 6/7, 2009 at 18:6 Comment(0)

On the question of "I still don't know the difference between Response.Close and CompleteRequest()" I would say:

Do prefer CompleteRequest(), don't use Response.Close().

See the following article for a well-done summary of this case.

Be aware that even after calling CompleteRequest() some text (e.g. redndered from ASPX code) would be appended to the response output stream. You can prevent it by overriding Render and RaisePostBackEvent methods as described in the following article.

BTW: I agree with preventing of using Response.End(), especially when writing data to the http stream to emulate file download. We've used the Response.End() in the past until our log file became full of ThreadAbortExceptions.

Cerous answered 16/9, 2010 at 11:58 Comment(2)

I'm interested in overridding Render as you describe, but the link to the "following article" is dead. Perhaps you can update your entry? – Hyperaemia 15/7, 2014 at 16:34

Sorry for a late answer. I do not exactly remember, what was in that article. However, I've found it on the webarchive.org: web.archive.org/web/20101224113858/http://www.c6software.com/… – Susette 18/8, 2014 at 12:15

I disagree with the statement "Response.End is harmful". It's definitely not harmful. Response.End does what it says; it ends execution of the page. Using reflector to see how it was implemented should only be viewed as instructive.

My 2cent Recommendation
AVOID using Response.End() as control flow.
DO use Response.End() if you need to stop request execution and be aware that (typically)* no code will execute past that point.

* Response.End() and ThreadAbortExceptions.

Response.End() throws a ThreadAbortException as part of it's current implementation (as noted by OP).

ThreadAbortException is a special exception that can be caught, but it will automatically be raised again at the end of the catch block.

To see how to write code that must deal with ThreadAbortExceptions, see @Mehrdad's reply to SO How can I detect a threadabortexception in a finally block where he references RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup Method and Constrained Execution Regions

The Rick Strahl article mentioned is instructive, and make sure to read the comments as well. Note that Strahl's issue was specific. He wanted to get the data to the client (an image) and then process hit-tracking database update that didn't slow down the serving of the image, which made his the problem of doing something after Response.End had been called.

Horripilate answered 7/7, 2009 at 22:13 Comment(1)

We saw this post #16732245 suggesting using Response.SuppressContent = True HttpContext.Current.ApplicationInstance.CompleteRequest() instead of Response.End() – Steviestevy 28/9, 2017 at 16:40

I've never considered using Response.End() to control program flow.

However Response.End() can be useful for example when serving files to a user.

You have written the file to the response and you don't want anything else being added to the response as it may corrupt your file.

Quizzical answered 6/7, 2009 at 16:49 Comment(1)

I understand the need for an API to say "the response is complete". But Response.End() also does a thread abort. This is the crux of the question. When is it a good idea to couple those two things? – Down 6/7, 2009 at 16:55

I've used Response.End() in both .NET and Classic ASP for forcefully ending things before. For instance, I use it when there is a certian amount of login attempts. Or when a secure page is being accesed from an unauthenticated login (rough example):

    if (userName == "")
    {
        Response.Redirect("......");
        Response.End();
    }
    else
    {
      .....

When serving files to a user I'd use a Flush, the End can cause issues.

Inflect answered 6/7, 2009 at 18:1 Comment(7)

Keep in mind, Flush() is not "this is the end". It's just "flush everything so far." The reason you might want a "this is the end" is to let the client be aware that it has all of the content, while the server can go and do other things - update a log file, query a database counter, or whatever. If you call Response.Flush and then do one of those things, the client may continue to wait for more. If you call Response.End() then control jumps out and the DB does not get queries, etc. – Down 7/7, 2009 at 3:41

You could alternatively use the override Response.Redirect("....", true) where the bool is 'endResponse: Indicates whether current execution of the page should terminate" – Horripilate 7/7, 2009 at 3:49

It's always better to use the Forms Authentication framework to protect pages that are meant to be secured by login credentials. – Horripilate 7/7, 2009 at 3:51

@Chesso, good to know. Currently I use it at the very end of a handler where I'm pulling image data from a database. So perhaps I'll revisit adding an End() below that. @Robert, I guess I never checked out Response.Redirect("....", true). Might be worth looking into. Thanks for the feedback. – Inflect 7/7, 2009 at 12:22

Actually, to correct myself, I believe the default of Response.Redirect and Server.Transfer are to call Response.End internally unless you call the override and pass 'false' in. The way your code is written the Response.End is never called, – Horripilate 7/7, 2009 at 22:15

Response.end works a lot differently in .net than it does in classic ASP. In .net it causes a threadabortexception, which can be pretty nasty. – Irritable 21/1, 2015 at 2:16

I think that Response.Close() is more appropriate for that scenario. It closes the connection to the client without aborting the current thread. – Corneous 22/5, 2020 at 2:52

I've only used Response.End() as a testing/debugging mechanism

<snip>
Response.Write("myVariable: " + myVariable.ToString());
Response.End();
<snip>

Judging from what you have posted in terms of research, I would say it would be a bad design if it required Response.End

Pharyngoscope answered 6/7, 2009 at 18:10 Comment(0)

On classic asp, I had a TTFB (Time To First Byte) of at 3 to 10 seconds on some ajax calls, much larger than the TTFB on regular pages with many more SQL calls.

The ajax returned was a segment of HTML to be injected into the page.

The TTFB was several seconds longer than the render time.

If I added a response.end after the render, the TTFB was greatly reduced.

I could get the same effect by emitting a "</body></html>", but this probably doesn't work when outputting json or xml; here response.end is needed.

Disherison answered 27/3, 2019 at 18:9 Comment(1)

I know this is an old answer, but then again, classic asp is old. I found it useful ;-) – Disherison 27/3, 2019 at 18:14

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