CODEWITHSUNDEEP
c#wpfuser-controls
M.Azad
M.Azad

Reputation: 3763

How to define a dependency property as reference type?

I have a user control that my user control has a DependencyProperty as a refrence type like Person:

public static readonly DependencyProperty MyPesonProperty =
    DependencyProperty.Register("Peson", typeof(Person), typeof(MyUserControl),
       new FrameworkPropertyMetadata
       {
           BindsTwoWayByDefault = true

       });

public Person MyPeson
{
   get { return (Person)GetValue(MyPesonProperty ); }
   set { 
            SetValue(MyPesonProperty , value);
       }
}

public MyUserControl()
{
        InitializeComponent();
        MyPeson= new Person();
}

public ChangePerson()
{
        MyPeson.FistName="B";
        MyPeson.LastName="BB";
}

when I call ChangePerson() I have a null reference exception for MyPerson property but I create a new instance from it in the constructor.

Upvotes: 1

Views: 754

Answers (1)

punker76
punker76

Reputation: 14611

i have no problems with your code. it works.

public partial class Window8 : Window
{
  public static readonly DependencyProperty MyPersonProperty =
    DependencyProperty.Register("MyPerson",
                                typeof(Person),
                                typeof(Window8),
                                new FrameworkPropertyMetadata(null, new PropertyChangedCallback(MyPersonPropertyChangedCallback)) {BindsTwoWayByDefault = true});

  private static void MyPersonPropertyChangedCallback(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e) {
    if (e.NewValue == null) {
      // ups, why is this null???
    }
  }

  public Person MyPerson {
    get { return (Person)this.GetValue(MyPersonProperty); }
    set { this.SetValue(MyPersonProperty, value); }
  }

  public Window8() {
    this.InitializeComponent();
    this.MyPerson = new Person();
  }

  private void Button_Click(object sender, RoutedEventArgs e) {
    // do something....
    this.MyPerson.FistName = "B";
    this.MyPerson.LastName = "BB";
  }
}

now, what can you do?

try debug and set a breakpoint to MyPersonPropertyChangedCallback and look what happens.

check you binding to MyPerson, perhaps the binding set this to null (combobox, selected item = null?)

hope this helps you...

Upvotes: 1

Related Questions