If you misspell an attribute name in an HTML document the browser will simply ignore it. For example, if you spell the href attribute as zhref the <a> tag the attribute will be ignored. For example, this is text is in such an <a> tag. It is displayed like a link but it doesn't go anywhere because it doesn't contain an href attribute. Intentional misspellings can be used to temporarily comment out an attributes or an element. XAML doesn't support this technique directly. If you misspell an attribute or an element the XAML parser will emit an error message and refused to produce a BAML file or load the document dynamically. This is the behavior you want if you unintentionally misspelled an attribute. The compiler will tell you exactly where the error is and you can quickly find and fix the problem. But temporarily commenting out an attribute can sometimes be useful when experimenting or prototyping. XAML provides a way to duplicate the this commenting out technique through the use of the markup compatibility namespace. Markup compatibility tells the parser which namespaces are not required to be recognized when loading a XAML document. If you declare a namespace that will never be recognized, such as "Comment", you can use that namespace to comment out items by using its prefix. For example, let say you want to see the effect of removing the Width property of a Button but not lose the value it currently has which would happen if you simply deleted it. Consider the following,

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"> 
  <Canvas>
    <Button Canvas.Top="50" Canvas.Left="50" Width="100" Height="50">
      Hello
    </Button>
  </Canvas>
</Window>

To comment out the Width attribute you can add the markup compatibility namespace, then an unknown namespace, and finally declare the unknown namespace ignorable. This might look like,

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:c="Comment"
    mc:Ignorable="c">
  <Canvas>
    <Button Canvas.Top="50" Canvas.Left="50" Width="100" Height="50">
      Hello
    </Button>
  </Canvas>
</Window>

You can then comment out the Width and Height attributes,

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:c="Comment"
    mc:Ignorable="c">
  <Canvas>
    <Button Canvas.Top="50" Canvas.Left="50" c:Width="100" c:Height="50">
      Hello
    </Button>
  </Canvas>
</Window>

You can even comment out the entire Button with,

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:c="Comment"
    mc:Ignorable="c">
  <Canvas>
    <c:Button Canvas.Top="50" Canvas.Left="50" Width="100" Height="50">
      Hello
    </c:Button>
  </Canvas>
</Window>

I only recommend this when you are prototyping. You shouldn't have commented out text in production XAML files. Fortunately, this technique makes it easy to determine if you have removed all such commented out items. Simply delete the xmlns:c and mc:Ignorable declarations from Window. The compiler will now generate and error if you haven't removed comment prefixes. Finally, this technique makes it clear to any reader (both mechanical and human) that you intended to comment out this property, you simple didn't just misspell it.

Mike Harsh and Mark Boulter demo'ed Cider for MSDN. Check it out...

Mark Boulter and Brian Pepin make an appearance on Channel 9 to explain and demo Cider. Check it out...

Brian Pepin has started to blog about Cider's architecture. Check it out...

If you missed the PDC and Eric Rudder's keynote address where Cider was demonstrated you can watch it here. Mark Boulter demonstrates it with Joe Marini demonstrating Sparkle. Their part begins around 37 minutes into the keynote.

Cider is now announced! In case you missed it at the PDC, Cider is the code name for the Visual Studio Designer for WPF (pronounced Avalon according to Chris) that will be delivered in Orcas. We also announced the Expression product line that includes a different designer called Sparkle. Other than the obvious difference between Sparkle and Cider, Cider is in VS, Sparkle is a stand-alone product, Cider will be a designer for developers, Sparkle will be a designer targeted at professional designers.

For those that were asking about what I was working on, I wasn't trying to be coy, I just didn't want to spill the beans early.

I will be more forthcoming in the months ahead and try to keep you up-to-date with our progress.

If I missed you at the PDC, I am sorry about that. Feel free to ask your questions here or via e-mail (chuckj directed through microsoft.com).