Some background:
The string started from "http:" in your sample is the namespace; and the "xmlns:x" introduce "x" as the associated keyword prefix you use to refer to the namespace throughout the XML code. The prefix itself can be anything (please see my comment to Valery's answer).
Name spaces is the way to introduce XML tags which can be world-unique (because URL can be world-unique and represent your organization and technology).
For XAML, this is the way to refer framework elements from different .NET name spaces, so there is mapping from .NET name spaces onto XML name spaces. Microsoft uses its own naming schema.
Try the following: choose some symbol unique to your current XAML, for example "
MyNameSpace
", go to XAML document tag (top-level) where attributes are and try to type the attribute:
xmlns:MyNameSpace=""
When you do it, VS UI will give you a choice of available name spaces you can put in your XAML. For example, you can use
xmlns:system="clr-namespace:System;assembly=mscorlib"
which maps .NET
System
onto XAML. This way, you can further use something like this:
<Window.Resources>
<ResourceDictionary>
<system:Double x:Key="SplitterSize">6</system:Double>
</Window.Resources>
...
Importantly, this
is also a way to insert your own control (framework element or whatever) in XAML. You develop you custom control code using some of your project name spaces, go to XAML and try to write a name space as I described above -- VS will show your namespace with your control; and you use it in your XAML.