C# 4 and .NET 4

744 ❘ ChaPTer 27 COre xAml
Xaml-defined markup extensions
Markup extensions provide a lot of capabilities. And indeed XAML-defined markup extensions have
already been used in this chapter. x:Array, which was shown in the “Collections” section, is defined as the
markup extension class ArrayExtension. With this markup extension, using the attribute syntax is not
possible because it would be difficult to define a list of elements.
Other markup extensions that are defined with XAML are the TypeExtension (x:Type), which returns the
type based on string input; NullExtension (x:Null), which can be used to set values to null in XAML; and
StaticExtension (x:Static) to invoke static members of a class.
WPF, WF, and WCF define markup extensions that are specific to these technologies. WPF uses markup
extensions to access resources, for data binding, and color conversion; WF uses markup extensions with
activities; and WCF defines markup extensions for endpoint definitions.
reading and WriTing Xaml
Several APIs exist for reading and writing XAML. There are high-level APIs that are easy to use but have
less functionality and low-level APIs with more features. Technology-specific APIs that make use of specific
WPF or WF features are also available. XAML can be read from a textual XML form, from BAML, or
from object trees, and written to XML or object trees.
Generic high-level APIs are available in the namespace System.Xaml. The class XamlServices allows
loading, parsing, saving, and transforming XAML. XamlServices.Load() can load XAML code from a
file, a stream, or a reader, or by using a XamlReader object. XamlReader (in the namespace System.Xaml)
is an abstract base class that has several concrete implementations. XamlObjectReader reads an object
tree, XamlXmlReader reads XAML from an XML file, Baml2006Reader reads the binary form of XAML.
XamlDebuggerXmlReader from the namespace System.Activities.Debugger is a special reader for WF
with special debugging support.
When passing XAML code in a string to XamlServices, the Parse() method can be used. XamlServices.
Save() can be used to save XAML code. With the Save() method you can use similar data sources as with
the Load() method. The object passed can be saved to a string, a stream, a TextWriter, a XamlWriter,
or an XmlWriter. XamlWriter is an abstract base class. Classes that derive from XamlWriter are
XamlObjectWriter and XamlXmlWriter. In the release of .NET 4, there’s no writer for BAML code, but it
is expected to have a BAML writer available at a later time.
When converting XAML from one format in another, you can use XamlServices.Transform(). With the
Transform method, you pass a XamlReader and a XamlWriter so that you can convert any format that is
supported by specific readers and writers.
Instead of using the high-level API XamlServices class, you can use generic low-level APIs directly, which
means using specific XamlReader and XamlWriter classes. With a reader, you can read node by node from a
XAML tree with the Read() method.
The generic XamlServices class doesn’t support specific WPF features such as dependency properties or
freezable objects. To read and write WPF XAML, you can use the classes XamlReader and XamlWriter
from the namespace System.Windows.Markup that is defined in the assembly PresentationFramework
and thus has access to the WPF features. The names of these classes might be confusing, as the same class
names are used with classes from different namespaces. System.Xaml.XamlReader is the abstract base
class for readers; System.Windows.Markup.XamlReader is the WPF class to read XAML. This can be even
more confusing when using the Load method of the WPF XamlReader class that accepts a System.Xaml
.XamlReader as argument.
An optimized version to read XAML for WF is WorkflowXamlServices in the namespace
System.Activities. This class is used to create dynamic activities during runtime.
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