CODEWITHSUNDEEP
wpfattached-properties
code4life
code4life

Reputation: 15794

Attached properties return null

I have the following attached property definition:

public class TestFocusManager
{
    public static readonly DependencyProperty FocusedElementProperty =
      DependencyProperty.RegisterAttached("FocusedElement", 
          typeof (UIElement), typeof(TestFocusManager));

    public static UIElement GetFocusedElement(DependencyObject obj)
    {
      return (UIElement) obj.GetValue(FocusedElementProperty);
    } 

    public static void SetFocusedElement(DependencyObject obj, UIElement value)
    {
      obj.SetValue(FocusedElementProperty, value);
    }
}

When I try to use it in my user control:

<UserControl xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:sys="clr-namespace:System;assembly=mscorlib" 
             mc:Ignorable="d" 
             xmlns:Behaviors="clr-namespace:MyLocalProject.Behaviors" 
             Behaviors:TestFocusManager.FocusedElement="{Binding ElementName=testElement}"
             x:Class="LocalProject.TestView"
             x:Name="_testView">
    <TextBox x:Name="testElement" />
</UserControl>

The attached property always returns a null... 

var result = TestFocusManager.GetFocusedElement(_testView); // <-- null...
var result2 = _testView.GetValue(TestFocusManager.FocusedElementProperty); // <-- again, null...

What am I doing wrong here? Thanks in advance!

Upvotes: 2

Views: 1093

Answers (3)

Sandepku
Sandepku

Reputation: 861

local:TestFocusManager.FocusedElement="{Binding RelativeSource={RelativeSource Self}}"

you can get a reference to the user control itself and expose a public property which gives out a button. You can use this as a last resort !.

Btw whenever I have had issues with attached properties. I generally tend to put a callback or change the type to that of Object, I mean instead of UIElement tend to use object atleast I get a callback and check whats the exact type comes as a part of callback

Cheers

Upvotes: 0

Adi Lester
Adi Lester

Reputation: 25201

Your problem is that GetFocusedElement is called before the binding is actually set (you're probably calling it in the UserControl's constructor). If you call it in the Loaded event, it should be fine.

Upvotes: 1

Trevor Elliott
Trevor Elliott

Reputation: 11252

I tested your code and it works fine for me except you omitted the "public" keyword in your Dependency Property setter.

I'm assuming that's a typo, if not then that's you're problem.

Upvotes: 0

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