Nested comments in XML?

Question

Are these nested comments allowed in a XML file?

<!-- Making only one observation attempting to correct the error code -->
<!-- <component>
       <!-- Result observation template -->
            <!-- <id root="2.16.840.1.113883.19.5.10" extension="103220"/>
     </component> -->

... wondering why noone mentioned using CDATA as a hacky block comment ... — Martin Ba
– Martin Ba, Commented Apr 18, 2016 at 8:36
@DavidBalažic - that's why I wrote "hacky" ... depending on what is processing the XML, the additional text node with all the cdata content might be ignored completely. Did I say that it is hacky? :-) — Martin Ba
– Martin Ba, Commented Dec 3, 2024 at 19:15

bluish · Accepted Answer · 2013-04-10 08:36:43Z

60

No, the string -- is not permitted to appear within comments in XML. So the fact you have -- show up inside another comment is going to cause failures.

And trying to post that answer also broke the text entry parsing ;)

For further proof, check the W3C specification:

http://www.w3.org/TR/2008/REC-xml-20081126/#sec-comments

The phrase

For compatibility, the string " -- " (double-hyphen) MUST NOT occur within comments.]

appears in the first paragraph of the section on XML comments.

edited Apr 10, 2013 at 8:36

bluish

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answered Aug 24, 2009 at 21:18

Brent Writes Code

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8 Comments

annakata Over a year ago

b w Over a year ago

System.Xml.XmlException: This is an invalid comment syntax.

Brent Writes Code Over a year ago

It's to ensure compatibility with SGML.

David Doria Over a year ago

@Brent Nash So how do you comment out a big block that has comments in it?

Manav Over a year ago

@BrentNash I'll take your word for it that it's just to ensure backward compatibility :). Still, for a format designed in the 90s, not allowing nested comments is not cool.

|

Community · Accepted Answer · 2017-05-23 12:34:43Z

51

As it is said in How do I comment out a block of tags in XML?, you can try to wrap your code with a non-existing processing-instruction, e.g.:

<?ignore
<component>
       <!-- Result observation template -->
            <!-- <id root="2.16.840.1.113883.19.5.10" extension="103220"/>
     </component> 
?>

edited May 23, 2017 at 12:34

CommunityBot

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answered Feb 18, 2016 at 10:36

psychoslave

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3 Comments

Dai Over a year ago

This is a great help for cases when badly-behaved NuGet packages mutilate my lovingly curated web.config files by adding their own elements and dozens of comments!

David Balažic Dec 2, 2024 at 11:07

We all know this is the real answer that OP and everyone landing here looked for.

Semperfi89 Jan 8 at 18:50

The only issue is, Visual Studo doesn't allow this. I get an error "Unrecognized element." when I run the web project. But I found out you can select the block and hit the "Comment" button. Everywhere there is a "" in front of it, and where there is a "-->" it will put "<!--" after it.

annakata · Accepted Answer · 2009-08-24 21:17:45Z

5

In a word - no.

The first encountered end-comment marker will, er... end the comment and the rest of it will look somewhat unpleasant from there on out.

answered Aug 24, 2009 at 21:17

annakata

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Comments

Philip Beck · Accepted Answer · 2018-11-18 00:54:05Z

5

Notepad++ together with the plugin XML Tools can do this.

Select a block of xml and on the xml tools submenu selected "Comment Selection".

Each existing "inner xml comment" will be altered so that it looks like this

  <!{1}** inner xml comment **{1}>

and if you add another outer comment in this way, those original inner comments will be further altered to

  <!{2}** inner xml comment **{2}>

answered Nov 18, 2018 at 0:54

Philip Beck

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Comments

Quentin · Accepted Answer · 2009-08-24 21:17:29Z

4

You can't. -- both starts and terminates a comment. This makes nesting them impossible.

answered Aug 24, 2009 at 21:17

Quentin

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Comments

Andrew D. Bond · Accepted Answer · 2022-09-07 04:25:01Z

0

Without a workaround described below, it's not allowed because -- can't appear in a comment.

Potential solutions:

answered Sep 7, 2022 at 4:25

Andrew D. Bond

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Comments

SailorDad · Accepted Answer · 2023-07-06 22:23:08Z

Here is how I accomplished the task. It's quick and dirty, but it shows the process. It parses properly and uses the NP++ Block comment/uncomment style

Python:

dummyCoordinates = [(0,0),(1000,0),(1000,10000),(0,10000),]

comment1 = ET.Comment("<CutoutSubdesign>")
xmlTag.insert(1, comment1)

comment2 = ET.Comment("\t<Polygon>")
xmlTag.insert(2, comment2)

idxCount = 3

for X,Y in dummyCoordinates:
    comment = ET.Comment("\t\t<Point x=\"{:.3f}um\" y=\"{:.3f}um\"/>".format(X,Y))
    xmlTag.insert(idxCount, comment)
    idxCount += 1

comment3 = ET.Comment("\t</Polygon>")
xmlTag.insert(idxCount, comment3)

comment4 = ET.Comment("</CutoutSubdesign>")
xmlTag.insert(idxCount + 1, comment4)

Result:

<!--<CutoutSubdesign>-->
<!--    <Polygon>-->
<!--        <Point x="0.000um" y="0.000um"/>-->
<!--        <Point x="1000.000um" y="0.000um"/>-->
<!--        <Point x="1000.000um" y="10000.000um"/>-->
<!--        <Point x="0.000um" y="10000.000um"/>-->
<!--    </Polygon>-->
<!--</CutoutSubdesign>-->

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