XAML overview

 Microsoft introduce the XAML language and XAML concepts to the Windows Store app developer audience, and describe the different ways to declare objects and set attributes in XAML as it is used for creating a Windows Store app.

What is XAML?

Extensible Application Markup Language (XAML) is a declarative language. Specifically, XAML can initialize objects and set properties of objects, using a language structure that shows hierarchical relationships between multiple objects, and using a backing type convention that supports extension of types. You can create visible user interface (UI)elements in the declarative XAML markup. You can then associate a separate code-behind file for each XAML file that can respond to events and manipulate the objects that you originally declare in XAML. The XAML language supports interchange of sources between different tools and roles in the development process without information loss, such as exchanging XAML sources between Visual Studio and Microsoft Expression Blend so that designer roles and developer roles are separate and can iterate on the development of an app. XAML files are XML files that generally have the .xaml file name extension.

Basic XAML Syntax : 

XAML has a basic syntax that builds on XML, and by definition valid XAML must be valid XML. But XAML also has syntax concepts that are assigned a different and more complete meaning. For example, it supports property element syntax, where property values can be set through elements rather than strings. 

Microsoft Visual Studio helps you to produce valid XAML syntax, both in the XAML text editor and in the XAML design surface. So, don't worry too much about the syntax with each keystroke. The IDE often helps you write valid syntax via autocompletion, suggestions in Microsoft IntelliSense lists, or other techniques. If this is your first experience with XAML, it might still be useful to know the syntax rules and particularly the terminology that is sometimes used to describe the restrictions or choices.

XAML namespaces :

By a broad definition for programming, a namespace determines how strings or identifiers that reference programming entities are interpreted. A namespace can also resolve ambiguities if you reuse identifiers. By using namespaces, a programming framework can separate user-declared identifiers from framework-declared identifiers, disambiguate identifiers through namespace qualifications, and so on. A XAML namespace serves this purpose for the XAML language. Here's how XAML applies and extends XML namespace concepts:

• XAML uses the reserved default-namespace XML attribute xmlns for namespace declarations, and the value of the attribute is a URI.
• XAML uses prefix declarations to declare non-default namespaces, and prefix usages in elements and attributes to reference that namespace.
• XAML has a concept of default namespace, which doesn't have to be the XML language default namespace.
• Namespace definitions inherit in an XML document from parent element to child element.
• Attributes of an element inherit the element's namespaces.                                                

XAML file almost always declares a default XAML namespace in its root element. The default XAML namespace defines which elements you can be declare without qualifying them by a prefix.

The XAML language XAML namespace : 

One particular XAML namespace that is declared in nearly every Windows Runtime XAML file is the XAML namespace for elements that are defined by the XAML language. By convention, the XAML language XAML namespace is mapped to the prefix "x". The default project and file templates for Windows Store app projects always define both the default XAML namespace (no prefix, just xmlns=) and the XAML language namespace (prefix "x") as part of the root element. For example, this snippet is a template-created Page root of the initial page for an app (showing the opening tag only, and simplified): 

<Page
    x:Class="Application1.BlankPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>
</Page>

The "x" prefix/XAML language XAML namespace contains several programming constructs that you use often in your XAML. Here are the most common 

x: prefix/XAML namespace constructs:

x:key : Sets a unique user-defined key for each resources in a ResourceDictionay. The Key          token string is the argument for the StaticResource markup extension to retrieve any such resource from another XAML usage.
x:Class : pecifies the code namespace and class name for the class that provides code-behind for a XAML page, and names the class that is created or joined by the build actions that support the XAML markup compiler when the app is compiled. You must have such a class to support code-behind, or to have your XAML content be initialized as Window.Content.
x:Name :Specifies a run-time object name for the instance that exists in run-time code after an object element defined in XAML is processed. You can always use x:Name for element identification, which is useful when FrameworkElement.Name is not supported on that element type.                                                                                                                       x:Uid : Identifies elements that should use localized resources for some of their property values.