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---
# Dependency Property Metadata
The [!INCLUDE[TLA#tla_winclient](../../../includes/tlasharptla-winclient-md.md)] property system includes a metadata reporting system that goes beyond what can be reported about a property through reflection or general common language runtime (CLR) characteristics. Metadata for a dependency property can also be assigned uniquely by the class that defines a dependency property, can be changed when the dependency property is added to a different class, and can be specifically overridden by all derived classes that inherit the dependency property from the defining base class.
The Windows Presentation Foundation (WPF) property system includes a metadata reporting system that goes beyond what can be reported about a property through reflection or general common language runtime (CLR) characteristics. Metadata for a dependency property can also be assigned uniquely by the class that defines a dependency property, can be changed when the dependency property is added to a different class, and can be specifically overridden by all derived classes that inherit the dependency property from the defining base class.
<a name="prerequisites"></a>
## Prerequisites
This topic assumes that you understand dependency properties from the perspective of a consumer of existing dependency properties on [!INCLUDE[TLA#tla_winclient](../../../includes/tlasharptla-winclient-md.md)] classes, and have read the [Dependency Properties Overview](dependency-properties-overview.md). In order to follow the examples in this topic, you should also understand [!INCLUDE[TLA2#tla_xaml](../../../includes/tla2sharptla-xaml-md.md)] and know how to write [!INCLUDE[TLA2#tla_winclient](../../../includes/tla2sharptla-winclient-md.md)] applications.
This topic assumes that you understand dependency properties from the perspective of a consumer of existing dependency properties on WPF applications.
<a name="dp_metadata_contents"></a>
## How Dependency Property Metadata is Used
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<a name="override_or_subclass"></a>
## When to Override Metadata, When to Derive a Class
The [!INCLUDE[TLA2#tla_winclient](../../../includes/tla2sharptla-winclient-md.md)] property system has established capabilities for changing some characteristics of dependency properties without requiring them to be entirely re-implemented. This is accomplished by constructing a different instance of property metadata for the dependency property as it exists on a particular type. Note that most existing dependency properties are not virtual properties, so strictly speaking "re-implementing" them on inherited classes could only be accomplished by shadowing the existing member.
The WPF property system has established capabilities for changing some characteristics of dependency properties without requiring them to be entirely re-implemented. This is accomplished by constructing a different instance of property metadata for the dependency property as it exists on a particular type. Note that most existing dependency properties are not virtual properties, so strictly speaking "re-implementing" them on inherited classes could only be accomplished by shadowing the existing member.
If the scenario you are trying to enable for a dependency property on a type cannot be accomplished by modifying characteristics of existing dependency properties, it might then be necessary to create a derived class, and then to declare a custom dependency property on your derived class. A custom dependency property behaves identically to dependency properties defined by the [!INCLUDE[TLA2#tla_winclient](../../../includes/tla2sharptla-winclient-md.md)] APIs. For more details about custom dependency properties, see [Custom Dependency Properties](custom-dependency-properties.md).
If the scenario you are trying to enable for a dependency property on a type cannot be accomplished by modifying characteristics of existing dependency properties, it might then be necessary to create a derived class, and then to declare a custom dependency property on your derived class. A custom dependency property behaves identically to dependency properties defined by the WPF APIs. For more details about custom dependency properties, see [Custom Dependency Properties](custom-dependency-properties.md).
One notable characteristic of a dependency property that you cannot override is its value type. If you are inheriting a dependency property that has the approximate behavior you require, but you require a different type for it, you will have to implement a custom dependency property and perhaps link the properties through type conversion or other implementation on your custom class. Also, you cannot replace an existing <xref:System.Windows.ValidateValueCallback>, because this callback exists in the registration field itself and not within its metadata.
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### Overriding Metadata
The purpose of overriding metadata is primarily so that you have the opportunity to change the various metadata-derived behaviors that are applied to the dependency property as it exists on your type. The reasons for this are explained in more detail in the [Metadata](#dp_metadata_contents) section. For more information including some code examples, see [Override Metadata for a Dependency Property](how-to-override-metadata-for-a-dependency-property.md).
Property metadata can be supplied for a dependency property during the registration call (<xref:System.Windows.DependencyProperty.Register%2A>). However, in many cases, you might want to provide type-specific metadata for your class when it inherits that dependency property. You can do this by calling the <xref:System.Windows.DependencyProperty.OverrideMetadata%2A> method. For an example from the [!INCLUDE[TLA2#tla_winclient](../../../includes/tla2sharptla-winclient-md.md)] APIs, the <xref:System.Windows.FrameworkElement> class is the type that first registers the <xref:System.Windows.UIElement.Focusable%2A> dependency property. But the <xref:System.Windows.Controls.Control> class overrides metadata for the dependency property to provide its own initial default value, changing it from `false` to `true`, and otherwise re-uses the original <xref:System.Windows.UIElement.Focusable%2A> implementation.
Property metadata can be supplied for a dependency property during the registration call (<xref:System.Windows.DependencyProperty.Register%2A>). However, in many cases, you might want to provide type-specific metadata for your class when it inherits that dependency property. You can do this by calling the <xref:System.Windows.DependencyProperty.OverrideMetadata%2A> method. For an example from the WPF APIs, the <xref:System.Windows.FrameworkElement> class is the type that first registers the <xref:System.Windows.UIElement.Focusable%2A> dependency property. But the <xref:System.Windows.Controls.Control> class overrides metadata for the dependency property to provide its own initial default value, changing it from `false` to `true`, and otherwise re-uses the original <xref:System.Windows.UIElement.Focusable%2A> implementation.
When you override metadata, the different metadata characteristics are either merged or replaced.
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This behavior is implemented by <xref:System.Windows.PropertyMetadata.Merge%2A>, and can be overridden on derived metadata classes.
#### Overriding Attached Property Metadata
In [!INCLUDE[TLA2#tla_winclient](../../../includes/tla2sharptla-winclient-md.md)], attached properties are implemented as dependency properties. This means that they also have property metadata, which individual classes can override. The scoping considerations for an attached property in [!INCLUDE[TLA2#tla_winclient](../../../includes/tla2sharptla-winclient-md.md)] are generally that any <xref:System.Windows.DependencyObject> can have an attached property set on them. Therefore, any <xref:System.Windows.DependencyObject> derived class can override the metadata for any attached property, as it might be set on an instance of the class. You can override default values, callbacks, or WPF framework-level characteristic-reporting properties. If the attached property is set on an instance of your class, those override property metadata characteristics apply. For instance, you can override the default value, such that your override value is reported as the value of the attached property on instances of your class, whenever the property is not otherwise set.
In WPF, attached properties are implemented as dependency properties. This means that they also have property metadata, which individual classes can override. The scoping considerations for an attached property in WPF are generally that any <xref:System.Windows.DependencyObject> can have an attached property set on them. Therefore, any <xref:System.Windows.DependencyObject> derived class can override the metadata for any attached property, as it might be set on an instance of the class. You can override default values, callbacks, or WPF framework-level characteristic-reporting properties. If the attached property is set on an instance of your class, those override property metadata characteristics apply. For instance, you can override the default value, such that your override value is reported as the value of the attached property on instances of your class, whenever the property is not otherwise set.
> [!NOTE]
> The <xref:System.Windows.FrameworkPropertyMetadata.Inherits%2A> property is not relevant for attached properties.