How can I comment a single line in XML?

It is the same as the HTML or JavaScript block comments:

<!-- The to-be-commented XML block goes here. -->

Not orthodox, but it works for me sometimes; set your comment as another attribute:

<node usefulAttr="foo" comment="Your comment here..."/>

No, there is no way to comment a line in XML and have the comment end automatically on a linebreak.

XML has only one definition for a comment:

'<!--' ((Char - '-') | ('-' (Char - '-')))* '-->'

XML forbids -- in comments to maintain compatibility with SGML.

As others have said, there is no way to do a single line comment legally in XML that comments out multiple lines, but, there are ways to make commenting out segments of XML easier.

Looking at the example below, if you add '>' to line one, the XmlTag will be uncommented. Remove the '>' and it's commented out again. This is the simplest way that I've seen to quickly comment/uncomment XML without breaking things.

<!-- --
<XmlTag variable="0" />
<!-- -->

The added benefit is that you only manipulate the top comment, and the bottom comment can just sit there forever. This breaks compatibility with SGML and some XML parsers will barf on it. So long as this isn't a permanent fixture in your XML, and your parsers accept it, it's not really a problem.

Stack Overflow's and Notepad++'s syntax highlighter treat it like a multi-line comment, C++'s Boost library treats it as a multi-line comment, and the only parser I've found so far that breaks is the one in .NET, specifically C#. So, be sure to first test that your tools, IDE, libraries, language, etc. accept it before using it.

If you care about SGML compatibility, simply use this instead:

<!-- -
<XmlTag variable="0" />
<!- -->

Add '->' to the top comment and a '-' to the bottom comment. The downside is having to edit the bottom comment each time, which would probably make it easier to just type in <!-- at the top and --> at the bottom each time.

I also want to mention that other commenters recommend using an XML editor that allows you to right-click and comment/uncomment blocks of XML, which is probably preferable over fancy find/replace tricks (it would also make for a good answer in itself, but I've never used such tools. I just want to make sure the information isn't lost over time). I've personally never had to deal with XML enough to justify having an editor fancier than Notepad++, so this is totally up to you.