How to add comments into a Xaml file in WPF?

I used this syntax as I found online but it throws an error:

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
<!-- Cool comment -->
xmlns:System="clr-namespace:System;assembly=mscorlib"

> 'Name cannot begin with the '<' character, hexadecimal value 0x3C. > Line 4, position 5.' XML is not valid.

I assume those XAML namespace declarations are in the parent tag of your control? You can't put comments inside of another tag. Other than that, the syntax you're using is correct.

<UserControl xmlns="...">
    <!-- Here's a valid comment. Notice it's outside the <UserControl> tag's braces -->
    [..snip..]
</UserControl>

Found a nice solution by Laurent Bugnion, it can look something like this:

<UserControl xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:comment="Tag to add comments"
             mc:Ignorable="d comment" d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <Button Width="100"
                comment:Width="example comment on Width, will be ignored......">
        </Button>
    </Grid>
</UserControl>

Here's the link: http://blog.galasoft.ch/posts/2010/02/quick-tip-commenting-out-properties-in-xaml/

A commenter on the link provided extra characters for the ignore prefix in lieu of highlighting:

mc:Ignorable=”ØignoreØ”

You can't insert comments inside xml tags.

Bad

<Window xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
<!-- Cool comment -->
xmlns:System="clr-namespace:System;assembly=mscorlib">

Good

<Window xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:System="clr-namespace:System;assembly=mscorlib">
<!-- Cool comment -->

Just a tip:

In Visual Studio to comment a text, you can highlight the text you want to comment, and then use Ctrl + K followed by Ctrl + C. To uncomment, you can use Ctrl + K followed by Ctrl + U.

You cannot put comments inside UWP XAML tags. Your syntax is right.

TO DO:

<xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:System="clr-namespace:System;assembly=mscorlib"/>
<!-- Cool comment -->

NOT TO DO:

<xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    <!-- Cool comment -->
xmlns:System="clr-namespace:System;assembly=mscorlib"/>

For anyone learning this stuff, comments are more important, so drawing on Xak Tacit's idea
(from User500099's link) for Single Property comments, add this to the top of the XAML code block:

<!--Comments Allowed With Markup Compatibility (mc) In XAML!
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:ØignoreØ="http://www.galasoft.ch/ignore"
    mc:Ignorable="ØignoreØ"
    Usage in property:
ØignoreØ:AttributeToIgnore="Text Of AttributeToIgnore"-->

Then in the code block

<Application FooApp:Class="Foo.App"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:ØignoreØ="http://www.galasoft.ch/ignore"
mc:Ignorable="ØignoreØ"
...

AttributeNotToIgnore="TextNotToIgnore"
...

...
ØignoreØ:IgnoreThisAttribute="IgnoreThatText"
...   
>
</Application>