C# 4 and .NET 4

742 ❘ ChaPTer 27 COre xAml
This resource can be referenced by using the StaticResourceExtension with attribute syntax to set the
Background property of a TextBlock. Attribute syntax is defined by curly brackets and the name of
the extension class without the Extension postfix.
The longer form of the attribute shorthand notation uses element syntax, as the next code snippet
demonstrates. StaticResourceExtension is defined as a child element of the TextBlock.Background
element. The property ResourceKey is set with an attribute to gradientBrush1. In the previous example,
the resource key is not set with the property ResourceKey (which would be possible as well) but with a
constructor overload where the resource key can be set.
Creating Custom markup extensions
A markup extension is created by defining a class that derives from the base class MarkupExtension. Most
markup extensions have the Extension postfix (this naming convention is similar to the Attribute postfix
with attributes, which you can read about in Chapter 14, “Reflection”). With a custom markup extension,
you only need to override the method ProvideValue(), which returns the value from the extension. The
type that is returned is annotated to the class with the attribute MarkupExtensionReturnType. With the
method ProvideValue, an IServiceProvider object is passed. With this interface you can query for
different services, such as IProvideValueTarget or IXamlTypeResolver. IProvideValueTarget can be
used to access the control and property where the markup extension is applied to with the TargetObject
and TargetProperty properties. IXamlTypeResolver can be used to resolve XAML element names to
CLR objects. The custom markup extension class CalculatorExtension defines the properties X and Y
of type double and an Operation property that is defined by an enumeration. Depending on the value of
the Operation property, different calculations are done on the X and Y input properties, and a string is
returned.
using System;
using System.Windows;
using System.Windows.Markup;
namespace Wrox.ProCSharp.XAML
{
public enum Operation
{
Add,
Subtract,
Multiply,
Divide
}
[MarkupExtensionReturnType(typeof(string))]
public class CalculatorExtension : MarkupExtension
{
public CalculatorExtension()
{
}
public double X { get; set; }
public double Y { get; set; }
public Operation Operation { get; set; }
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