EDIT
So I managed to fix it by defining an Application_Error method in my Global.asax and call Response.ClearContent from there. Not sure if this could have any undesirable side-effects though, so still interested in answers to the original question. /EDIT
I have set up the <customErrors> element in my Web.config using redirectMode="ResponseRewrite". According to MSDN this should display the error page without changing the original URL.
This works fine if an exception is thrown in one of the page events like Page_Load, however I noticed when you throw an exception from an inline code-block in your .aspx file, it will output whatever you had in your .aspx file up to the line where the exception is raised and then append the contents of your error page to that, resulting in total garbage on the client's end.
Take this simple page, with a Web.config
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.web>
<customErrors mode="On" defaultRedirect="~/error.aspx" redirectMode="ResponseRewrite" />
</system.web>
</configuration>
and error.aspx page
<%@ Page Language="C#" AutoEventWireup="true" %>
<html>
<head></head>
<body>
An error occured
</body>
</html>
and a default.aspx that raises an exception
<%@ Page Language="C#" AutoEventWireup="true" %>
<% string s = null; %>
<html>
<head></head>
<body>
<strong>This is a test page.</strong>
<%= s[0] %>
</body>
</html>
Requesting default.aspx in the client's browser will result in the following garbled HTML being served
<html>
<head></head>
<body>
<strong>This is a test page.</strong>
<html>
<head></head>
<body>
An error occured
</body>
</html>
Does this happen because at this stage the HTML of default.aspx file has already been written to the output stream? If so, is there anyway to clear the contents of the output stream so that only the contents of the actual error.aspx will be send to the client? I tried calling Response.ClearContent() in the Page_Load of my error page, but that had no effect.
Is there anything I can do to prevent this from happening other than resorting to redirectMode="ResponseRedirect"? Also, if what I wrote above is true, how come redirectMode="ResponseRedirect" works in this situation? If the output stream has already been written, HTTP headers must already have been sent, right?