Why does XmlDocument.GetElementById always return null?

StackOverflow https://stackoverflow.com/questions/3775785

  •  04-10-2019
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Question

I've got some XML (valid XHTML) that looks like this:

<html>
    <head>
        <script type="text/javascript">
            <![CDATA[
                function change_header(){
                    document.getElementById("myHeader").innerHTML="Nice day!";
                }]]>
        </script>
    </head>
    <body>
        <h1 id="myHeader">Hello World!</h1>
        <button onclick="change_header()">Change text</button>
    </body>
</html>

And I'm trying to get the #myHeader node using docment.GetElementById("myHeader") but it always returns null. Why?

I'm guessing it doesn't recognize the id attribute as the id attribute without a DTD or something? If that's the case, how can I get it to use an HTML DTD?

Was it helpful?

Solution

It's because XmlDocument knows nothing about what an id means. You need to include a DTD in your XHTML document. Just put the following in the beginning of your html file:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

Example:

string html = @"<!DOCTYPE html PUBLIC ""-//W3C//DTD XHTML 1.0 Transitional//EN"" ""http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd""><html><body><div id=""foo"">some content</div></body></html>";
XmlDocument document = new XmlDocument();
document.LoadXml(html);
XmlElement div = document.GetElementById("foo");

Notice that this might be a little slower because the DTD needs to be downloaded.

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