How does "DependencyProperty" work?

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How does "DependencyProperty" work?

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#1 by (4 votes)

1

I'm trying to understand the dependency properties feature. I have read some tutorials in msdn but the feature is still obscure for 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'm not understanding the "mechanics" of how methods work for the UserControl1 class. That is, at what time they are being called, and by whom.

c# wpf xaml

asked by anonymous 03.01.2017 / 17:46

1 answer

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score 4 · Accepted Answer

For a property of a UserControl ( DependencyObject ) can be used in XAML and thus can 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 registered in the " WPF property system ".

This is done using the static% method of the DependencyProperty class .

A Dependency Property is made up of two "parts":

Property identifier - An instance of DependencyProperty obtained by returning the Register() method and saved as a static member of DependencyObject . This identifier is used as a parameter for many of the APIs that interact with the "WPF property system" . The agreement requires that your name be the property plus DependencyProperty.Register()
```
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 Property and GetValue() of DependencyObject to implement the "get" and "set" of the property.
```
public string SetText 
{
     get {return(string) GetValue(SetTextProperty); }
     set {SetValue(SetTextProperty, value);}
}
```

The SetValue() method has 3 overloads :

public static DependencyProperty Register (String name, Type propertyType, Type ownerType) - Register a dependency property with the given name, its type and its owner type. This is the minimum required to register the property.
public static DependencyProperty Register (string name, Type propertyType, Type ownerType, PropertyMetadata typeMetadata) - Register a dependency property with the given name, type, owner type, and metadata.
public static DependencyProperty Register (string name, Type propertyType, Type typeType, PropertyMetadata typeMetadata, ValidateValueCallback validateValueCallback) - Register a dependency property with the given name, its type, its owner type, metadata, and a callback for value validation.

When registering the property, in addition to indicating the name, type and type of its owner it is possible to indicate several callbacks ,

These callbacks are:

PropertyChangedCallback - Via PropertyMetadata , is called when the value of the property changes.
CoerceValueCallback - Via PropertyMetadata, is called when the value to be evaluated.
ValidateValueCallback - Is called after the value of the property be attributed. You should return DependencyProperty.Register() or true according to validation.

So, by answering your question, these methods are being called by the "WPF property system" while interacting with the property.

More information on Dependency Properties Overview . >