Question

When I start a postback using __doPostBack, a file is created and going back to the user to download in the HttpContext.Current.Response.

Because I change the Response, the page including its javascript values is not modified

But when I have no file to output, the page is refreshed (because of the postback) and the javascript modification on the page are lost.

How can I ‘stop’ the postback from continuing and persist my current page? I can’t use an async postback, because I need the postback to let the user download the file.

EDIT: more info after some questions in the comments:

• The file is requested in a webservice request. The webservice needs
  to execute a heavy query to determine if a file will be created. I
  prefer that this only happens once.
• The user can drag / drop some filters that will be used in the file
  request. If no file is available, the user should be able to change
  his filters, so thats why the page should not be changed.

Answer 1

From the W3 standards and RFC 2616:

10.2.5 204 No Content The server has fulfilled the request but does not need to return an entity-body, and might want to return updated
metainformation. The response MAY include new or updated
metainformation in the form of entity-headers, which if present SHOULD
be associated with the requested variant.
If the client is a user agent, it SHOULD NOT change its document view from that which caused the request to be sent. This response is
primarily intended to allow input for actions to take place without
causing a change to the user agent’s active document view, although
any new or updated metainformation SHOULD be applied to the document
currently in the user agent’s active view. The 204 response MUST NOT
include a message-body, and thus is always terminated by the first
empty line after the header fields.

Note the bolded line here. I have not tried it myself; however, setting the HTTP status to 204 and sending back an empty document, rather than trying to stop postback entirely, is certainly worth a shot.

Good luck, I hope this helps.

EDIT: this is the code that does the trick:
System.Web.HttpContext.Current.Response.StatusCode = 204;