How does the Dependency Property (Dependencyproperty) "work"?

Question

I’m trying to understand the feature of dependency properties. I’ve read some tutorials on msdn but the feature is still unclear to me.

Example:

using System; 
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication3 {

   /// <summary> 
      /// Interaction logic for UserControl1.xaml 
   /// </summary> 

   public partial class UserControl1 : UserControl {

      public UserControl1() {
         InitializeComponent();
      }

      public static readonly DependencyProperty
         SetTextProperty = DependencyProperty.Register("SetText", typeof(string), 
         typeof(UserControl1), new PropertyMetadata("", new PropertyChangedCallback(OnSetTextChanged)));

      public string SetText {
         get {return(string) GetValue(SetTextProperty); }
         set {SetValue(SetTextProperty, value);}
      }

      private static void OnSetTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
         UserControl1 UserControl1Control = d as UserControl1;
         UserControl1Control.OnSetTextChanged(e);
      }

      private void OnSetTextChanged(DependencyPropertyChangedEventArgs e) {
         tbTest.Text = e.NewValue.ToString();
      }
   }
}       

XAML

<Window x:Class = "WpfApplication3.MainWindow"
   xmlns = "http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x = "http://schemas.microsoft.com/winfx/2006/xaml"
   xmlns:views = "clr-namespace:WpfApplication3" 
   Title = "MainWindow" Height = "350" Width = "604">

   <Grid>
      <views:UserControl1 SetText = "Hellow World" />
   </Grid>

</Window>

I don’t understand the "mechanics" of the operation of class methods UserControl1. That is, at what time they are being called, and by whom.

Answer 1

So that a property of a Usercontrol(Dependencyobject) can be used in XAML and thus be accessed/calculated by means other than the traditional "get" and "set", such as themes, Binding, animations, templates, etc, it must be declared as Dependencyproperty and recorded in "WPF Property system".

This is done using the static method Register() class Dependencyproperty.

One Dependency Property is composed of two "parts":

• Property identifier - An instance of Dependencyproperty obtained by the return of the method DependencyProperty.Register() and kept as a static member of the Dependencyobject. This identifier is used as a parameter for many of the Apis that interact with the "WPF Property system".
  The convention requires that your name be the property plus Property

  public static readonly DependencyProperty SetTextProperty = 
      DependencyProperty.Register("SetText", typeof(string), typeof(UserControl1),
               new PropertyMetadata("", new PropertyChangedCallback(OnSetTextChanged)));
  

• CLR "wrapper" - The actual implementation of the property. This implementation uses the identifier and methods GetValue() and SetValue() of Dependencyobject to implement the property’s "get" and "set".

  public string SetText 
  {
       get {return(string) GetValue(SetTextProperty); }
       set {SetValue(SetTextProperty, value);}
  }
  

The method DependencyProperty.Register() has 3 overloads:

• public Static Dependencyproperty Register(String name, Type propertyType, Type ownerType) - Records a dependency Property with the given name, its type and type of its owner. This is the minimum required to register the property.

• public Static Dependencyproperty Register(string name,Type propertyType,Type ownerType,Propertymetadata typeMetadata) - Records a dependency Property with the given name, its type, type of its owner and metadata.

• public Static Dependencyproperty Register(string name,Type propertyType,Type ownerType,Propertymetadata typeMetadata, Validatevaluecallback validateValueCallback) - Records a dependency Property with the given name, its type, its owner’s type, metadata and a callback for value validation.

When registering the property, you can also enter its owner’s name, type and type callbacks,

Those callbacks sane:

• Propertychangedcallback - Via Propertymetadata, is called when the property value is changed.
• Coercevaluecallback - Via Propertymetadata, is called when the value is being evaluated.
• Validatevaluecallback - It is called after the value of the property is assigned. It must return true or false according to the validation.

So, in answer to your question, these methods are being called by "WPF Property system" while interacting with property.

More information on Dependency Properties Overview.