== Dependency Properties == If data binding is required to certain properties, then these need to be implemented as Dependency Properties. Here is a good example (from: [[http://csharpsimplified.wordpress.com/2008/12/26/dependency-property|C# is Easy: Dependency Property]]):


public class Button : ButtonBase
{
  // The dependency property
  public static readonly DependencyProperty IsCancelProperty;
  
  // Constructor
  static Button()
  {
    // Register the property
    Button.IsCancelProperty = DependencyProperty.Register(“IsCancel”,
        typeof(bool), typeof(Button),
        new FrameworkPropertyMetadata(false,
        new PropertyChangedCallback(OnIsCancelPropertyChanged)));
    …
  }
  
  // A .NET property wrapper (not must)
  public bool IsCancel
  {
    get { return (bool)GetValue(Button.IsCancelProperty ); }
    set { SetValue(Button.IsCancelProperty , value); }
  }
  
  // A property changed callback (not must)
  private static void OnIsCancelPropertyChanged(
    DependencyObject o, DependencyPropertyChangedEventArgs e) { … }
  …
}

Alternatively:


public class Button : ButtonBase
{
  // The dependency property declaration, and registration
  public static readonly DependencyProperty IsCancelProperty = DependencyProperty.Register(“IsCancel”,
        typeof(bool), typeof(Button),
        new FrameworkPropertyMetadata(false,
        new PropertyChangedCallback(OnIsCancelPropertyChanged)));
        
  // A .NET property wrapper (not must)
  public bool IsCancel
  {
    get { return (bool)GetValue(Button.IsCancelProperty ); }
    set { SetValue(Button.IsCancelProperty , value); }
  }
  
  // A property changed callback (not must)
  private static void OnIsCancelPropertyChanged(
    DependencyObject source, DependencyPropertyChangedEventArgs e) 
  {
       Button control = source as Button;
 
       // Put some update logic here...
       control.IsCancel = (bool)e.NewValue;
       if (control.IsCancel == true)
       {
           // do something
       } 
  }
  
  // Constructor
  static Button()
  {
  }
  
  …
}

IMPORTANT: The accessor property is fine to use, but the XAML code will only call the SetValue() and GetValue() methods directly, so make sure that no other code gets added to the accessor property, to make sure both work the same. === Example 1 ===


public class Employee : DependencyObject
{
    private static readonly DependencyProperty EmpIdProperty =
        DependencyProperty.Register("EmpId", typeof(int), typeof(Employee),
        new PropertyMetadata(0,
            new PropertyChangedCallback(EmpIdCallBack),
            new CoerceValueCallback(CoerceEmpId)));

    private static readonly DependencyProperty NameProperty =
        DependencyProperty.Register("Name", typeof(string), typeof(Employee));

    private static readonly DependencyProperty DepartmentProperty =
        DependencyProperty.Register("Department", typeof(string), typeof(Employee));

    private static readonly DependencyProperty CompanyProperty =
        DependencyProperty.Register("Company", typeof(string), typeof(Employee),
        new PropertyMetadata("WPF In Corp"));

    // If employee id is greater than 1000 then make it 1000
    static void EmpIdCallBack(DependencyObject d, DependencyPropertyChangedEventArgs args)
    {
        Employee emp = (Employee)d;

        if (emp.EmpId >= 1000)
        {
            emp.EmpId = 1000;
        }
    }

    // If the employee Id is negative then make it positive
    static Object CoerceEmpId(DependencyObject d, Object baseValue)
    {
        int value = (int)baseValue;

        if (value < 0)
            return value * -1;
        else
            return value;
    }

    public int EmpId
    {
        get { return (int)GetValue(EmpIdProperty); }
        set { SetValue(EmpIdProperty, value); }
    }

    public string Name
    {
        get { return (string)GetValue(NameProperty); }
        set { SetValue(NameProperty, value); }
    }

    public string Department
    {
        get { return (string)GetValue(DepartmentProperty); }
        set { SetValue(DepartmentProperty, value); }
    }

    public string Company
    {
        get { return (string)GetValue(CompanyProperty); }
        set { SetValue(CompanyProperty, value); }
    }
}

Source: [[http://zamjad.wordpress.com/2009/07/13/dependency-property-metadata|Zeeshan Amjad: Dependency Property Metadata]] === Example 2 === A user control that requires a Dependency Property in order to set the ''SelectedIndex'' in a ''ListBox'' using Data Binding. XAML:



. . .

C# code:


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
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 ACME.Controls
{
    public partial class ucActiveInstrument : UserControl
    {
        #region DependencyProperty SelectedItemIndex
        //-----------------------------------
        // SelectedItemIndex
        //-----------------------------------
        // Dependency Property
        public static readonly DependencyProperty SelectedItemIndexProperty =
          DependencyProperty.Register(
              "SelectedItemIndex",
              typeof(int),
              typeof(ucActiveInstrument),
              new FrameworkPropertyMetadata(
                    new int(),
                    OnSelectedItemIndexPropertyChanged
              )
          );

        // .NET Property Wrapper
        public int SelectedItemIndex
        {
            // Important: Do not add any logic to these properties, because they are only 
            // called when you set the property from code. If you set the property from XAML 
            // the SetValue() method is called directly.
            get { return (int)GetValue(SelectedItemIndexProperty); }
            set { SetValue(SelectedItemIndexProperty, value); }
        }
        // Alias
        public int SelectedItemIdx
        {
            get { return SelectedItemIndex; }
        }

        ///----------------------------------------------------------------------------------------
        /// 
        /// OnPropertyChanged event handler for fitting view model.
        /// 
        /// Source control
        /// Event Arguments
        ///----------------------------------------------------------------------------------------
        private static void OnSelectedItemIndexPropertyChanged(
          DependencyObject source,
          DependencyPropertyChangedEventArgs e)
        {
            ucActiveInstrument control = source as ucActiveInstrument;
            //DateTime time = (DateTime)e.NewValue;

            //-----------------------------------------
            // Put some update logic here...
            //-----------------------------------------
            //SetDataBinding((TFittingViewModel)e.NewValue, control);
            //SetBinauralView(control);
            control.lstOptions.SelectedIndex = (int)e.NewValue;
        }
        #endregion

        //-----------------------------------------------------------------------------------------
        /// 
        /// Constructor.
        /// 
        //-----------------------------------------------------------------------------------------
        public ucActiveInstrument()
        {
            InitializeComponent();
        }

        //-----------------------------------------------------------------------------------------
        /// 
        /// SelectionChange event handler for lstOptions.
        /// 
        /// 
        /// 
        //-----------------------------------------------------------------------------------------
        private void lstOptions_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            this.SelectedItemIndex = lstOptions.SelectedIndex;
        }

    }
}

== See Also == * [[swdev:dotnet:wpf:implementing_mvvm|Implementing M-V-VM]] * [[http://zamjad.wordpress.com/2009/07/13/dependency-property-metadata/|Zeeshan Amjad: Dependency Property Metadata]]