What is xmlns in every WPF file?

xmlns is an XML, not necessarily XAML, construct which defines a namespace in which to resolve xml element names. Because it is defined without a qualifier, it is defining the default namespace by which an XML element name should be resolved.

In XAML you usually see the following entry. It defines the default namespace to be essentially WPF and all XML element names are hence resolved as WPF elements.

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

It's also common to see non-default namespaces such as the following.

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

This defines a qualified namespace for XAML specific elements. If you want an element or attribute name to be resolved within this namespace you should qualify it with x. For example

<StackPanel x:Name="foo" />

There are 2 name resolutions in this definition.

1. StackPanel - Because it's an unqualified name, it will be resolved in the default namespace which is WPF
2. x:Name - Name is qualified with x and will be resolved within the XAML document.

And you use xmlns to get a reference to your own namespaces within your XAML as well. One of the first things I do when creating a new WPF project is to add a reference to the project namespace:

xmlns:local="clr-namespace:MyWpfProject"

Now I have access to any classes I may create within my project (like IValueConverters and DataTemplateSelectors) by using the "local:" prefix

<local:BooleanToColorConverter x:Key="booleanToColorConverter" DefaultBrush="Green" HighlightBrush="Red" />

Of course, you don't have to use "local", you can name it whatever you want. And you can add references to any other namespace you need the same way.

You can also map multiple CLR namespaces together into one XML namespace by adding XmlnsDefinitionAttribute to your assemblies. This is what the WPF team did, by mapping a lot of namespaces under System.Windows like this:

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

This syntax can simplify your XAML, but be careful not to have classes with the same name in the CLR namespaces you merge together.