Xaml what is a dependency property

One important reason is that you cannot set triggers on normal. NET propeties. Creating a read only property is similar to creating a regular DependencyProperty.

Instead of calling DependencyProperty. Register you call DependencyProperty. This returns you a DependencyPropertyKey. This key should be stored in a private or protected static readonly field of your class. The key gives you access to set the value from within your class and use it like a normal dependency property. Second thing to do is registering a public dependency property that is assigned to DependencyPropertyKey. This property is the readonly property that can be accessed from external.

Attached properties are a special kind of DependencyProperties. They allow you to attach a value to an object that does not know anything about this value. A good example for this concept are layout panels. Each layout panel needs different data to align its child elements.

Since you can write your own layout panel, the list is infinite. So you see, it's not possible to have all those properties on all WPF controls. The solution are attached properties. They are defined by the control that needs the data from another control in a specific context. For example an element that is aligned by a parent layout panel. To set the value of an attached property, add an attribute in XAML with a prefix of the element that provides the attached property.

To set the the Canvas. Top and Canvas. Left property of a button aligned within a Canvas panel, you write it like this:. If you want to listen to changes of a dependency property, you can subclass the type that defines the property and override the property metadata and pass an PropertyChangedCallback. Because null is also a valid local value, there is the constant DependencyProperty. UnsetValue that describes an unset value.

Home Fundamentals Dependency Properties. NET 4. FromProperty TextBox. TextProperty , typeof TextBox ; if textDescr! ClearValue Button. ContentProperty ;. Feed About Christian Moser. Dependency properties solve these problems by only store modified properties in the instance. The default values are stored once within the dependency property. Value inheritance When you access a dependency property the value is resolved by using a value resolution strategy.

If no local value is set, the dependency property navigates up the logical tree until it finds a value. When you set the FontSize on the root element it applies to all textblocks below except you override the value. Change notification Dependency properties have a built-in change notification mechanism.

By registering a callback in the property metadata you get notified, when the value of the property has been changed. This is also used by the databinding. Value resolution strategy Every time you access a dependency property, it internally resolves the value by following the precedence from high to low. Comments on this article Show all comments. Commented on This property will be picked by the DependencyPropertyAttribute aspect and set to the proper DependencyProperty value.

The following example shows how to expose the DependencyProperty for the Phone property. PostSharp Code Contracts see Contracts provide a convenient way to validate the values of the dependency properties. To add the validation to your dependency property, you just need to apply a contract attribute to that property.

The following code snippet shows how to validate a dependency property using a code contract. If you need more complex validation for your dependency property, you can implement it in a dedicated validation method. To define a validation method for the Email dependency property, add a new method named ValidateEmail to the same class where the property is declared. The method must accept one argument with the type assignable from the property type and return a bool value.

The following list shows the method signatures you can use when implementing the validation method where TPropertyType is the type of the dependency property and TDeclaringType is the class where your property is declared. The following code snippet shows how to validate a dependency property using a validation method. The WPF property system can automatically notify you about the dependency property value changes via callback methods.

This can be useful when, for example, you need to update the visual presentation of your custom UI control in response to a change of its property. This section shows how you can define a property change callback method with the PostSharp's dependency property pattern.

To define a property change callback method for the PictureUrl dependency property, add a new method named OnPictureUrlChanged to the same class where the property is declared. The method doesn't have to accept any arguments and must have a void return type. Implement your property change handling logic inside this new method. The following list shows the method signatures you can use when implementing the property change callback method. TDeclaringType is the class where your property is declared.

You may also want to notify the users of your class when a dependency property value changes. In this case, you would normally need to implement the INotifyPropertyChanged interface in your class and raise the PropertyChanged event. Dependency object is the base object for all WPF objects. Dependency objects are used for WPF property system. So DependencyObject implements all the plumbing for the sytyling, databinding, animation systems and more that is transparent to the developer, and Dependency Properties are the access points to store Set and retrieve Get the associated data.

Implementation is a little bit more involved than standard properties however, as mentioned above, can do far more. To implement, it is a two-part process:. Luckily, Visual Studio makes this easy to do via code snippets. For C we have the propdb. Here is an example of the code auto-generated:. Dependency Properties can be used to extend functionality for existing controls lookless inheritence , adding properties to Usercontrols templates , Behaviors, and Markup Extensions to add or modify existing controls without inheritance.

For the purpose of this article, I am going to inherit the TextBox control and add a Valid input state. See image below:.