Custom dependency properties

Applies to Windows and Windows Phone

Here we explain how to define and implement your own dependency properties for a Windows Runtime app using C++, C#, or Visual Basic. We list reasons why app developers and component authors might want to create custom dependency properties. We describe the implementation steps for a custom dependency property, as well as some best practices that can improve performance, usability, or versatility of the dependency property.

Prerequisites

We assume that you have read the Dependency properties overview and that you understand dependency properties from the perspective of a consumer of existing dependency properties. To follow the examples in this topic, you should also understand XAML and know how to write a basic Windows Runtime app using C++, C#, or Visual Basic.

What is a dependency property?

Dependency properties are properties that are registered with the Windows Runtime property system by calling the DependencyProperty.Register method, and that are identified by a DependencyProperty identifier member on the defining class. You can enable what would otherwise be a common language runtime (CLR) or C++ property to support styling, data binding, animations, and default values by implementing it as a dependency property. Dependency properties can be used only by DependencyObject types. But DependencyObject is quite high in the class hierarchy, so the majority of classes that are intended for UI and presentation support can support dependency properties. For more information about dependency properties and some of the terminology and conventions used for describing them in this documentation, see Dependency properties overview.

Examples of dependency properties in the Windows Runtime are: Control.Background, FrameworkElement.Width, and TextBox.Text, among many others. Each dependency property exposed by a class has a corresponding public static readonly property of type DependencyProperty that is exposed on that same class and that is the identifier for the dependency property. The identifier's name follows this convention: the name of the dependency property, with the string "Property" added to the end of the name. For example, the corresponding DependencyProperty identifier for the Control.Background property is Control.BackgroundProperty. The identifier stores the information about the dependency property as it was registered, and the identifier can then be used later for other operations involving the dependency property, such as calling SetValue.

Property wrappers

Dependency properties typically have a wrapper implementation. Without the wrapper, the only way to get or set the properties would be to use the dependency property utility methods GetValue and SetValue and to pass the identifier to them as a parameter. This is a rather unnatural usage for something that is ostensibly a property. But with the wrapper, your code and any other code that references the dependency property can use a straightforward object-property syntax that is natural for the language you're using.

If you implement a custom dependency property yourself and want it to be public and easy to call, define the property wrappers too. The property wrappers are also useful for reporting basic information about the dependency property to reflection or static analysis processes. Specifically, the wrapper is where you place attributes such as ContentPropertyAttribute.

When to implement a property as a dependency property

Whenever you implement a public read/write property on a class, as long as your class derives from DependencyObject, you have the option to make your property work as a dependency property. Sometimes the typical technique of backing your property with a private field is adequate. Defining your custom property as a dependency property is not always necessary or appropriate. The choice will depend on the scenarios that you intend your property to support.

You might consider implementing your property as a dependency property when you want it to support one or more of these features of the Windows Runtime or of Windows Runtime apps:

  • Setting the property through a Style
  • Acting as valid target property for data binding
  • Supporting animated values through a Storyboard
  • Reporting when the previous value of the property has been changed by:
    • Actions taken by the property system itself
    • The environment
    • User actions
    • Reading and writing styles

Checklist for defining a dependency property

Defining a dependency property can be thought of as a set of concepts. These concepts are not necessarily procedural steps, because several concepts can be addressed in a single line of code in the implementation. This list gives just a quick overview. We'll explain each concept in more detail later in this topic, and we'll show you example code in several languages.

  • (Optional) Create property metadata for the dependency property. You need property metadata only if you want property-changed behavior, or a metadata-based default value that can be restored by calling ClearValue.

  • Register the property name with the property system (call Register), specifying an owner type and the type of the property value. There's a required parameter for Register that expects property metadata. Specify null for this, or specify the actual property metadata if you have declared any.

  • Define a DependencyProperty identifier as a public static readonly property member on the owner type.

  • Define a wrapper property, following the property accessor model that's used in the language you are implementing. The wrapper property name should match the name string that you used in Register. Implement the get and set accessors to connect the wrapper with the dependency property that it wraps, by calling GetValue and SetValue and passing your own property's identifier as a parameter.
  • (Optional) Place attributes such as ContentPropertyAttribute on the wrapper.

Note  If you are defining a custom attached property, you generally omit the wrapper. Instead, you write a different style of accessor that a XAML processor can use. See Custom attached properties.

Registering the property

For your property to be a dependency property, you must register the property into a property store maintained by the Windows Runtime property system. You must give the property a unique identifier to be used as the qualifier for later property-system operations. These operations might be internal operations, or your own code calling property-system APIs. To register the property, you call the Register method.

For Microsoft .NET languages (C# and Microsoft Visual Basic) you call Register within the body of your class (inside the class, but outside any member definitions). The identifier is also provided by the Register method call, as the return value. The Register call is typically made outside of other member definitions because you use the return value to assign and create a public static readonly property of type DependencyProperty as part of your class. This property becomes the identifier for your dependency property. Here are examples of the Register call.

No code example is currently available or this language may not be supported.

Note  Registering the dependency property in a class body is the typical implementation, but you can also register a dependency property in the class static constructor. This approach may make sense if you need more than one line of code to initialize the dependency property.

For C++, you have options for how you split the implementation between the header and the code file. The typical split is to declare the identifier itself as public static property in the header, with a get implementation but no set. The get implementation refers to a private field, which is an uninitialized DependencyProperty instance. You can also declare the wrappers and the get and set implementations of the wrapper. In this case the header includes some minimal implementation. If the wrapper needs Windows Runtime attribution, attribute in the header too. Put the Register call in the code file, within a helper function that only gets run when the app initializes the first time. Use the return value of Register to fill the static but uninitialized identifiers that you declared in the header, which you initially set to nullptr at the root scope of the implementation file.


//.h file
//using namespace Windows::UI::Xaml::Controls;
//using namespace Windows::UI::Xaml::Interop;
//using namespace Windows::UI::Xaml;
//using namespace Platform;

public ref class ImageWithLabelControl sealed : public Control
{  
private:
    static DependencyProperty^ _LabelProperty;
...
public:
    static void RegisterDependencyProperties(); 
    static property DependencyProperty^ LabelProperty
    {
        DependencyProperty^ get() {return _LabelProperty;}
    }
...
};



//.cpp file
using namespace Windows::UI::Xaml;
using namespace Windows::UI::Xaml.Interop;

DependencyProperty^ ImageWithLabelControl::_LabelProperty = nullptr;

// This function is called from the App constructor in App.xaml.cpp 
// to register the properties
void ImageWithLabelControl::RegisterDependencyProperties() 
{ 
    if (_LabelProperty == nullptr) 
    { 
        _LabelProperty = DependencyProperty::Register(
          "Label", Platform::String::typeid, ImageWithLabelControl::typeid, nullptr); 
    } 
}

Note  For the C++ code, the reason why you have a private field and a public read-only property that surfaces the DependencyProperty is so that other callers who use your dependency property can also use property-system utility APIs that require the identifier to be public. If you keep the identifier private, people can't use these utility APIs. Examples of such API and scenarios include GetValue or SetValue by choice, ClearValue, GetAnimationBaseValue, SetBinding, and Setter.Property. You can't use a public field for this, because Windows Runtime compile rules don't allow public data members that use reference types like DependencyProperty.

Dependency property name conventions

There are naming conventions for dependency properties; follow them in all but exceptional circumstances. The dependency property itself has a basic name ("Label" in the preceding example) that is given as the first parameter of Register. The name must be unique within each registering type, and the uniqueness requirement also applies to any inherited members. Dependency properties inherited through base types are considered to be part of the registering type already; names of inherited properties cannot be registered again.

Caution  Although the name you provide here can be any string identifier that is valid in programming for your language of choice, you usually want to be able to set your dependency property in XAML too. To be set in XAML, the property name you choose must be a valid XAML name. For more info, see XAML overview.

When you create the identifier property, combine the name of the property as you registered it with the suffix "Property" ("LabelProperty", for example). This property is your identifier for the dependency property, and it is used as an input for the SetValue and GetValue calls you make in your own property wrappers. It is also used by the property system and potentially by XAML processors.

Implementing the wrapper

Your property wrapper should call GetValue in the get implementation, and SetValue in the set implementation.

Caution  In all but exceptional circumstances, your wrapper implementations should perform only the GetValue and SetValue operations. Otherwise, you'll get different behavior when your property is set via XAML versus when it is set via code. For efficiency, the XAML parser bypasses wrappers when setting dependency properties; whenever possible, it uses the registry of dependency properties.



//using namespace Platform;
public:
...
  property String^ Label
  {
    String^ get() {
      return (String^)GetValue(LabelProperty);
    }
    void set(String^ value) {
      SetValue(LabelProperty, value); 
    }
  }

Property metadata for a custom dependency property

When property metadata is assigned to a dependency property, the same metadata is applied to that property for any instance of the property-owner type or its subclasses. In property metadata, you can specify two behaviors:

  • A default value that the property system assigns to all cases of the property.
  • A static callback method that is automatically invoked within the property system whenever a property value is detected.

Calling Register with property metadata

In the previous examples of calling DependencyProperty.Register, we passed a null value for the propertyMetadata parameter. To enable a dependency property to provide a default value or use a property-changed callback, you must define a PropertyMetadata instance that provides one or both of these capabilities.

Typically you provide a PropertyMetadata as an inline-created instance, within the parameters for DependencyProperty.Register.

Note  If you are defining a CreateDefaultValueCallback implementation, you must use the utility method PropertyMetadata.Create rather than calling a PropertyMetadata constructor to define the PropertyMetadata instance.

This next example modifies the previously shown DependencyProperty.Register examples by referencing a PropertyMetadata instance with a PropertyChangedCallback value. The implementation of the "OnLabelChanged" callback will be shown later in this section.


DependencyProperty^ ImageWithLabelControl::_LabelProperty = 
    DependencyProperty::Register("Label", 
    Platform::String::typeid,
    ImageWithLabelControl::typeid, 
    ref new PropertyMetadata(nullptr,
      ref new PropertyChangedCallback(&ImageWithLabelControl::OnLabelChanged))
    );

Default value

You can specify a default value for a dependency property such that the property always returns a particular default value when it is unset. This value can be different than the inherent default value for the type of that property.

If a default value is not specified, the default value for a dependency property is null for a reference type, or the default of the type for a value type or language primitive (for example, 0 for an integer or an empty string for a string). The main reason for establishing a default value is that this value is restored when you call ClearValue on the property. Establishing a default value on a per-property basis might be more convenient than establishing default values in constructors, particularly for value types. However, for reference types, make sure that establishing a default value does not create an unintentional singleton pattern. For more info, see Best practices later in this topic

Note  Do not register with a default value of UnsetValue. If you do, it will confuse property consumers and will have unintended consequences within the property system.

CreateDefaultValueCallback

In some scenarios, you are defining dependency properties for objects that are used on more than one UI thread. This might be the case if you are defining a data object that is used by multiple apps, or a control that you use in more than one app. You can enable the exchange of the object between different UI threads by providing a CreateDefaultValueCallback implementation rather than a default value instance, which is tied to the thread that registered the property. Basically a CreateDefaultValueCallback defines a factory for default values. The value returned by CreateDefaultValueCallback is always associated with the current UI CreateDefaultValueCallback thread that is using the object.

To define metadata that specifies a CreateDefaultValueCallback, you must call PropertyMetadata.Create to return a metadata instance; the PropertyMetadata constructors do not have a signature that includes a CreateDefaultValueCallback parameter.

The typical implementation pattern for a CreateDefaultValueCallback is to create a new DependencyObject class, set the specific property value of each property of the DependencyObject to the intended default, and then return the new class as an Object reference via the return value of the CreateDefaultValueCallback method.

Property-changed callback method

You can define a property-changed callback method to define your property's interactions with other dependency properties, or to set an internal property or state of your object whenever the property changes. If your callback is invoked, the property system has determined that there is an effective property value change. Because the callback method is static, the d parameter of the callback is important because it tells you which instance of the class has reported a change. A typical implementation uses the NewValue property of the event data and processes that value in some manner, usually by performing some other change on the object passed as d. Additional responses to a property change are to reject the value reported by NewValue, to restore OldValue, or to set the value to a programmatic constraint applied to the NewValue.

This next example shows a PropertyChangedCallback implementation. It implements the method you saw referenced in the previous Register examples, as part of the construction arguments for the PropertyMetadata. The scenario addressed by this callback is that the class also has a calculated read-only property named "HasLabelValue" (implementation not shown). Whenever the "Label" property gets reevaluated, this callback method is invoked, and the callback enables the dependent calculated value to remain in synchronization with changes to the dependency property.


static void OnLabelChanged(DependencyObject^ d, DependencyPropertyChangedEventArgs^ e)
{
    ImageWithLabelControl^ iwlc = (ImageWithLabelControl^)d;
    Platform::String^ s = (Platform::String^)(e->NewValue);
    if (s->IsEmpty()) {
        iwlc->HasLabelValue=false;
    }
}

Property changed behavior for structures and enumerations

If the type of a DependencyProperty is an enumeration or a structure, the callback may be invoked even if the internal values of the structure or the enumeration value did not change. This is different from a system primitive such as a string where it only is invoked if the value changed. This is a side effect of box and unbox operations on these values that is done internally. If you have a PropertyChangedCallback method for a property where your value is an enumeration or structure, you need to compare the OldValue and NewValue by casting the values yourself and using the overloaded comparison operators that are available to the now-cast values. Or, if no such operator is available (which might be the case for a custom structure), you may need to compare the individual values. You would typically choose to do nothing if the result is that the values have not changed.


static void OnVisibilityValueChanged(DependencyObject^ d, DependencyPropertyChangedEventArgs^ e)
{
    if ((Visibility)e->NewValue != (Visibility)e->OldValue)
    {
        //value really changed, invoke your changed logic here
    } 
    // else this was invoked because of boxing, do nothing
    }
}

Best practices

Keep the following considerations in mind as best practices when as you define your custom dependency property.

DependencyObject and threading

All DependencyObject instances must be created on the UI thread which is associated with the current Window that is shown by a Windows Runtime app. Although each DependencyObject must be created on the main UI thread, the objects can be accessed using a dispatcher reference from other threads, by calling Dispatcher.

The threading aspects of DependencyObject are relevant because it generally means that only code that runs on the UI thread can change or even read the value of a dependency property. Threading issues can usually be avoided in typical UI code that makes correct use of async patterns and background worker threads. You typically only run into DependencyObject-related threading issues if you are defining your own DependencyObject types and you attempt to use them for data sources or other scenarios where a DependencyObject isn't necessarily appropriate.

Avoiding unintentional singletons

An unintentional singleton can happen if you are declaring a dependency property that takes a reference type, and you call a constructor for that reference type as part of the code that establishes your PropertyMetadata. What happens is that all usages of the dependency property share just one instance of PropertyMetadata and thus try to share the single reference type you constructed. Any subproperties of that value type that you set through your dependency property then propagate to other objects in ways you probably don't intend.

You can use class constructors to set initial values for a reference-type dependency property if you want a non-null value, but be aware that this would be considered a local value for purposes of dependency property precedence. It might be more appropriate to use a template for this purpose, if your class supports templates. Another way to avoid a singleton pattern, but still provide a useful default, is to expose a static property on the reference type that provides a suitable default for the values of that class.

Collection-type dependency properties

Collection-type dependency properties have some additional implementation issues to consider.

Collection-type dependency properties are relatively rare in the Windows Runtime API. In most cases, you can use collections where the items are a DependencyObject subclass, but the collection property itself is implemented as a conventional CLR or C++ property. This is because collections do not necessarily suit some typical scenarios where dependency properties are involved. For example:

  • You do not typically animate a collection.
  • You do not typically prepopulate the items in a collection with styles or a template.
  • Although binding to collections is a major scenario, a collection does not need to be a dependency property to be a binding source. For binding targets, it is more typical to use subclasses of ItemsControl or DataTemplate to support collection items, or to use view-model patterns. For more info about binding to and from collections, see Data binding with XAML.
  • Notifications for collection changes are better addressed through interfaces such as INotifyPropertyChanged or INotifyCollectionChanged, or by deriving the collection type from ObservableCollection.

Nevertheless, scenarios for collection-type dependency properties do exist. The next three sections provide some guidance on how to implement a collection-type dependency property.

Initializing the collection

When you create a dependency property, you can establish a default value by means of dependency property metadata. But be careful to not use a singleton static collection as the default value. Instead, you must deliberately set the collection value to a unique (instance) collection as part of class-constructor logic for the owner class of the collection property.

Change notifications

Defining the collection as a dependency property does not automatically provide change notification for the items in the collection by virtue of the property system invoking the "PropertyChanged" callback method. If you want notifications for collections or collection items—for example, for a data-binding scenario— implement the INotifyPropertyChanged or INotifyCollectionChanged interface. For more info, see Data binding with XAML.

Dependency property security considerations

Declare dependency properties as public properties. Declare dependency property identifiers as public static read-only members. Even if you attempt to declare other access levels permitted by a language (such as protected), a dependency property can always be accessed through the identifier in combination with the property-system APIs. Declaring the dependency property identifier as internal or private will not work, because then the property system cannot operate properly.

Wrapper properties are really just for convenience, Security mechanisms applied to the wrappers can be bypassed by calling GetValue or SetValue instead. So keep wrapper properties public; otherwise you just make your property harder for legitimate callers to use without providing any real security benefit.

The Windows Runtime does not provide a way to register a custom dependency property as read-only.

Dependency properties and class constructors

There is a general principle that class constructors should not call virtual methods. This is because constructors can be called to accomplish base initialization of a derived class constructor, and entering the virtual method through the constructor might occur when the object instance being constructed is not yet completely initialized. When you derive from any class that already derives from DependencyObject, remember that the property system itself calls and exposes virtual methods internally as part of its services. To avoid potential problems with run-time initialization, don't set dependency property values within constructors of classes.

Registering the dependency properties for C++/CX apps

The implementation for registering a property in C++/CX is trickier than C#C#, both because of the separation into header and implementation file and also because initialization at the root scope of the implementation file is a bad practice. (Visual C++ component extensions (C++/CX) puts static initializer code from the root scope directly into DllMain, whereas C# compilers assign the static initializers to classes and thus avoid DllMain load lock issues.). The best practice here is to declare a helper function that does all your dependency property registration for a class, one function per class. Then for each custom class your app consumes, you'll have to reference the helper registration function that's exposed by each custom class you want to use. Call each helper registration function once as part of the Application constructor (App::App()), prior to InitializeComponent. That constructor only runs when the app is really referenced for the first time, it won't run again if a suspended app resumes, for example. Also, as seen in the previous C++ registration example, the nullptr check around each Register call is important: it's insurance that no caller of the function can register the property twice. A second registration call would probably crash your app without such a check because the property name would be a duplicate. You can see this implementation pattern in the XAML user and custom controls sample if you look at the code for the C++/CX version of the sample.

Related topics

DependencyObject
DependencyProperty.Register
Dependency properties overview
XAML user and custom controls sample

 

 

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