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public static readonly DependencyProperty HeightProperty; // The DependencyProperty field is often registered // in the static constructor of the class. static FrameworkElement() { ... HeightProperty = DependencyProperty.Register( "Height", typeof(double), typeof(FrameworkElement), new FrameworkPropertyMetadata((double) 1.0 / (double) 0.0, FrameworkPropertyMetadataOptions.AffectsMeasure, new PropertyChangedCallback(FrameworkElement.OnTransformDirty)), new ValidateValueCallback(FrameworkElement.IsWidthHeightValid)); } // The CLR wrapper, which is implemented using // the inherited GetValue()/SetValue() methods. public double Height { get { return (double) base.GetValue(HeightProperty); } set { base.SetValue(HeightProperty, value); } } } As you can see, dependency properties require quite a bit of additional code from a normal CLR property! And in reality, a dependency can be even more complex than what you see here (thankfully, many implementations are simpler than Height). First and foremost, remember that if a class wants to define a dependency property, it must have DependencyObject in the inheritance chain, as this is the class which defines the GetValue() and SetValue() methods used in the CLR wrapper. Since FrameworkElement is-a DependencyObject, this requirement is satisfied. Next, recall that the entity which will hold the actual value of the property (a double in the case of Height) is represented as a public, static read-only field of type DependencyProperty. The name of this field should, by convention, always be named by suffixing the word Property to the name of the related CLR wrapper, like so: public static readonly DependencyProperty HeightProperty; Given that dependency properties are declared as static fields, they are typically created (and registered) within the static constructor of the class. The DependencyProperty object is created via a call to the static DependencyProperty.Register() method. This method has been overloaded many times; however, in the case of Height, DependencyProperty.Register() is invoked as follows: HeightProperty = DependencyProperty.Register( "Height", typeof(double), typeof(FrameworkElement), new FrameworkPropertyMetadata((double)0.0, FrameworkPropertyMetadataOptions.AffectsMeasure,

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new PropertyChangedCallback(FrameworkElementOnTransformDirty)), new ValidateValueCallback(FrameworkElementIsWidthHeightValid)); The first argument to DependencyPropertyRegister() is the name of the normal CLR property on the class (Height in this case), while the second argument is the type information of the underlying data type it is encapsulating (a double) The third argument specifies the type information of the class that this property belongs to (FrameworkElement in this case) While this might seem redundant (after all, the HeightProperty field is already defined within the FrameworkElement class), this is a very clever aspect of WPF in that it allows one class to register properties on another (even if the class definition has been sealed!) The fourth argument passed to DependencyPropertyRegister() in this example is what really gives dependency properties their own unique flavor.

Here a FrameworkPropertyMetadata object is passed that describes various details regarding how WPF should handle this property with respect to callback notifications (if the property needs to notify others when the value changes) and various options (represented by the FrameworkPropertyMetadataOptions enum) that control what is effected by the property in question (Does it work with data binding , Can it be inherited , etc) In this case, the constructor arguments of FrameworkPropertyMetadata break down as so: new FrameworkPropertyMetadata( // Default value of property (double)00, // Metadata options FrameworkPropertyMetadataOptionsAffectsMeasure, // Delegate pointing to method called when property changes new PropertyChangedCallback(FrameworkElementOnTransformDirty) ) Because the final argument to the FrameworkPropertyMetadata constructor is a delegate, note that its constructor parameter is pointing to a static method on the FrameworkElement class named OnTransformDirty().

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I won t bother to show the code behind this method, but be aware that any time you are building a custom dependency property, you can specify a PropertyChangedCallback delegate to point to a method that will be called when your property value has been changed This brings me to the final parameter passed to the DependencyPropertyRegister() method, a second delegate of type ValidateValueCallback, which points to a method on the FrameworkElement class which is called to ensure the value assigned to the property is valid: new ValidateValueCallback(FrameworkElementIsWidthHeightValid) This method contains logic you might normally expect to find in the set block of a property (more information on this point in the next section): private static bool IsWidthHeightValid(object value) { double num = (double) value; return ((!DoubleUtilIsNaN(num) && (num >= 00)) && !doubleIsPositiveInfinity(num)); }.

Once the DependencyProperty object has been registered, the final task is to wrap the field within a normal CLR property (Height in this case). Notice, however, that the get and set scopes do not simply return or set a class-level double-member variable, but do so indirectly using the GetValue() and SetValue() methods from the System.Windows.DependencyObject base class: public double Height { get { return (double) base.GetValue(HeightProperty); } set { base.SetValue(HeightProperty, value); } }