We Keep Coding

Server cannot set content type after HTTP headers have been sent on rowcommand

So I have a button inside my gridview. If the user clicks on the button a rowcommand is send and a pdf file is downloaded / written to the response.
Code
HttpResponse httpResponse = Response;
httpResponse.Clear();
httpResponse.ContentType = "application/pdf";
httpResponse.AddHeader("content-disposition", $"attachment;filename={rowData.Pod.fileName}");
httpResponse.Output.Write(bytes);
Response.Flush();
HttpContext.Current.ApplicationInstance.CompleteRequest();
For some reason I always get Server cannot set content type after HTTP headers have been sent. when tried to download the file. Now the gridview is in a updatepanel but that should not be a problem since other buttons for downloading work.
I have also tried to send the buffer to true before I did the file download but that did not change it.
EDIT
The problem was that there was still an async postback that I had setup on the 'selectedindexchanged' of the gridview to display a modal. Is there any way I can work around this? When I change the trigger the whole page reloads before showing the modal and also resprings. The file downloads after changing it to postback instead of asyncpostback

I had an issue that was extremely similar to this. I had a button inside a ListView control that was trying to print a pdf report. After extensive research I found a very simple solution.
Add OnInit="btnPrint_Init" to the button on the front of the page and then add the following to the code behind (change the name of the button to suit your needs):
protected void btnPrint_Init(object sender, EventArgs e)
{
Button btnPrint = (Button)sender;
ScriptManager.GetCurrent(this).RegisterPostBackControl(btnPrint);
}

Probably, Response.Buffer is false.
You should write the content after writing the headers.
httpResponse.ContentType = "application/pdf";
httpResponse.AddHeader("content-disposition", $"attachment;filename={rowData.Pod.fileName}");
httpResponse.Output.Write(bytes);
Actually, you can instead set a JavaScript command to those buttons requesting an http handler which is the best practice for downloading a file.
For security reasons you can create a guid and put the guid to session items for each row (having the file id as the value) and then send that guid to the http handler and retrieve the file id from session and response binary write the file

Postback Happening after Download when Clicking a Button or Link

This is a problem I've had in the past, but I forgot if I ever solved it or not. I have it so when the user clicks on an asp:linkbutton it triggers a download for a file. File downloads successfully without a problem. However, after it downloads if the user clicks to download again or clicks a button a postback happens and the page refreshes, clearing out everything such as tables or text. How do I prevent this from happening?
Here's the code executing for the download.
string name = Path.GetFileName(filePath);
Response.Clear();
Response.AddHeader("content-disposition", "attachment; filename=\"" + name + "\"");
Response.ContentType = mimeType;
Response.BinaryWrite(file);
Response.End();
NOTES:
The linkbutton control is NOT created behind the scenes in code, if that makes a difference.
The file does not download when clicking the link on the second time. Only a refresh happens.
This is part of a DNN module

First you need to save the information you want to keep before the post back happens. One of the ways this can be done is by using session. For example if you wanted to save the value of a text box you could say:
Session["TextBox1"] = TextBox1.Text;
Then you need to handle the post back in your page load function.
private void Page_Load()
{
//check if this is a post back
if(this.IsPostBack)
{
//restore your values
TextBox1.Text = (string)Session["TextBox1"];
}
}

Okay, I know the source of this problem , but I never found a solution for it. The reason this even shows up is because of a setting in DotNetNuke. Under Host Settings in the Advanced Settings tab if you look at Performance Settings there is a setting called Page State Persistence. If you set this to Memory like I did it will cause AJAX issues. It's even noted in the description. Setting it back to Page makes the problem disappear.
Reason I've kept it as memory is because my company's site was using Memory, however without my knowing it was switched back to Page. Now it's a non issue, but if anyone finds a solution for when it's set to Memory let me know! Otherwise, I'd suggest against using it unless it was fixed in newer versions of DNN.

Refresh page content after Response.End()

I have a piece of code that generates a csv file dynamically. The user clicks a button, I prepare the string and then I do so something like:
MemoryStream stream = new MemoryStream(ASCIIEncoding.Default.GetBytes(sb.ToString()));
Response.Clear();
Response.ContentType = "Application/csv";
Response.AppendHeader("content-disposition", String.Format("attachment; filename={0}.csv", fileName.ToString()));
Response.BinaryWrite(stream.ToArray());
Response.End();
The user is then prompted to download or save the newly generated file.
The problem is that after this I have to refresh some data in the page. After the Response.End this is not possible anymore.
Any ideas on how to overcome the problem?

This is a common problem and there are lots of ways to solve it which all boil down to needing to create two separate requests to your server. The first one will request your CSV and the second will request your refresh.
With this in mind it's obvious that you can either:
Get the user to create both requests (by clicking separately on two links)
Use javascript to create both when the user only clicks once
So for example the link that sets off your CSV download could also call a javascript function which causes your data to refresh. The usual way to do this would be to use OnClientClick to call some javascript which will refresh your data.
However you can decide the best way forward based on your app.

Maybe clicking on a button will in a new window redirect user to a handler (ashx) which will return the stream / prompt user to save the file?
-or-
Update content first, then send file to a user.

Redirecting to a "Thank you" page and offering the save dialog of downloaded file immediately

I am using ASP.NET2.0. I have created a download form with some input fields and a download button. When the download button is clicked, I want to redirect the user to a "Thank you for downloading..." page and immediately offer him/her the file to save.
I have this following code to show the savefile dialog:
public partial class ThankYouPage : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
Response.Clear();
Response.AddHeader("Content-Disposition",
"attachment; filename=\"downloadedFile.zip\"");
Response.ContentType = "application/x-zip-compressed";
Response.BinaryWrite(this.downloadedFileByteArray);
Response.Flush();
Response.End();
}
}
Obviously, this code does not allow to display any "Thank you" message. Is there an "AfterRender" event or something similar of the Page where, I could move this download code and give a chance for the page to render the "thank you" message to the user? After all, I am truely thankful to them, so I do want to express that.

You could reference a download page from your thank you page using an IFrame
<iframe src="DownloadFile.aspx" style="display:none;" />
In this case, DownloadFile.aspx would have the code behind from your example.

Use a META REFRESH tag in the head of your thank-you page:
<META http-equiv="refresh" content="1;URL=http://site.com/path/to/downloadedFile.zip">
Alternatively, you might use a body onLoad function to replace the current location with the download URL.
<body onLoad="document.location='http://site.com/path/to/downloadedFile.zip'">
In this case the redirection will start after the current page has finished loading and only if the client has JavaScript enabled, so remember to include a link with a download link ("If your download doesn't start in a few seconds..." and so on).
You may also use an IFRAME as suggested by Phil, or even a FRAME or a full-blown pop-up (blockable, mind you). Your mileage may vary.

The code you've written, should actually be redirected to from the 'thank you' page (making it the 2nd redirect). Because you've set the content disposition to attachment, this page will not actually replace the existing 'thank you' page.

if you want to serve a "Thank You" page and the file the client must call twice the server.
So you can just create the thankyou.aspx page for displaying the message (and maybe put a direct download link to the file).
You can start download with a meta tag or just using js (even ms do the same for their download page).
Then to serve the file you sould create a direct link to avoid another page run on the server; otherwise you should create an HttpHandler just to hide the filesys.
The file should be sent to the client with Response.TrasmitFile

Please check the following link, this may help you to sort out your issue.
In Asp.net, on button click, how to open a download dialog, and then redirect the user to another page?

Why do my Button send two postbacks when downloading zip file?

I've got a problem on a WebForms application where a user selects some criteria from drop downs on the page and hits a button on the page which calls this method:
protected void btnSearch_Click(object sender, EventArgs e)
They then click on button to download a zip file based on the criteria which calls this method:
protected void btnDownload_Click(object sender, EventArgs e)
In IE, they are prompted with the bar at the top of the browser that tells them:
"To help protect your security, Internet Explorer blocked this site
from downloading files to your computer".
When they click on that bar to download the file, it fires the btnSearch_Click event again.
Response.ContentType and Response.AddHeader has been set up correctly.
The problem is, that btnSearch appends criteria so basically it is being appended twice and causing problems.
Is there something I can do to prevent this?
This is a VS2008 web application using C# 3.5 for what it's worth.

When they click the download button, do a Redirect to the ZIP file handler (page?) to download the file. i.e. use the Post-Redirect-Get pattern: http://en.wikipedia.org/wiki/Post/Redirect/Get

Are you using the Content-Disposition header?
Response.AddHeader("Content-Disposition", "attachment; filename=fileName.zip");
Try changing the content type to match the file type?

Save a boolean value into the Session indicating that the criteria have been already appended.
When the user selects another value from the dropdowns then set this value to false.
Inside your btn_Download event you then can check the value of the Session variable and avoid setting the criteria twice.