In XAML, when you want to reference a CLR type, you have to add a namespace mapping that maps the XML namespace to the CLR namespace, like so:

xmlns:local="clr-namespace:MyTestApp.Controls;assembly=TestApp.Controls"

However the Types from PresentationFramework and PresentationCore are included differently, like so:

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

What I like about this declaration is the fact that it hides all the CLR-namespaces that are automatically imported. This is possible because inside PresentationFramework.dll (and some in PresentationCore.dll), we have a bunch of assembly attributes, namely the XmlnsDefinitionAttribute:

[assembly:
XmlnsDefinition("http://schemas.microsoft.com/winfx/2006/xaml/presentation",
"System.Windows")]

[assembly:
XmlnsDefinition("http://schemas.microsoft.com/winfx/2006/xaml/presentation",
"System.Windows.Data")]

[assembly:
XmlnsDefinition("http://schemas.microsoft.com/winfx/2006/xaml/presentation",
"System.Windows.Navigation")]

[assembly:
XmlnsDefinition("http://schemas.microsoft.com/winfx/2006/xaml/presentation",
"System.Windows.Shapes")]

[assembly:
XmlnsDefinition("http://schemas.microsoft.com/winfx/2006/xaml/presentation",
"System.Windows.Documents")]

[assembly:
XmlnsDefinition("http://schemas.microsoft.com/winfx/2006/xaml/presentation",
"System.Windows.Controls")]

This does the mapping from the CLR namespace to the XML namespace. So instead of having a list of xmlns mappings in your XAML file, you just have one, of the form xmlns:x=”http://schemas.microsoft.com/winfx/2006/xaml/presentation”. This not only simplifies the mapping declarations but it is also easier to remember. So say for example your company created a bunch of cool WPF controls; you could then have a XmlnsDefinitionAttribute in the Control Library as:

[assembly: XmlnsDefinition("http://www.my-company.com/wpf",
"MyCompany.Controls")]

Then your users could use these controls in their XAML files simply by including the namespace mapping:

xmlns:mc="http://www.my-company.com/wpf"

Now isn’t that a cooler way of declaring the XML namespace!