document.write in StringBuilder?

Question

I am using a StringBuilder in an ashx file to return javascript. It all works fine, except for one line...

javascript.Append("document.write(\"<script id=__ie_onload defer \" + \"src=javascript:void(0)><\/script>\");");

For readability, here is the unescaped version:

document.write("<script id=__ie_onload defer src=javascript:void(0)><\/script>");

If I comment out this line, my ashx file works like a charm! If I keep it in, though, it won't even run. Is there some sort of restriction on using a document.write statement in a StringBuilder? Alternatively, is there a better way to write out Javascript from within .NET? It has to be from .NET due to the need for using server variables.

Answer 1

You have an unrecognized escape sequence in your closing script tag. \/ is not a valid escape sequence. You probably want:

javascript.Append("document.write(\"<script id=__ie_onload defer \" + \"src=javascript:void(0)></script>\");");

Or if you really want that \/ sequence, then doubly-escape the \:

javascript.Append("document.write(\"<script id=__ie_onload defer \" + \"src=javascript:void(0)><\\/script>\");");

As a debugging tip, if your tools provide you with such poor feedback that you can't even see compilation errors, try creating a test application containing the problematic code. I copied your code into a console application, something like:

static void Main(string[] args)
{
    var javascript = new StringBuilder();
    javascript.Append(...
    Console.Write(javascript);
}

...and I saw the problem immediately on compilation:

Unrecognized escape sequence

...with the problem sequence underlined.