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Data binding attached property to view model fails

I created an attached property for ListBox like this:

using ListBoxControl = System.Windows.Controls.ListBox;

namespace App.Ui.Views.AttachedProperties

public class ListBox

public static readonly DependencyProperty autoScrollProperty =
DependencyProperty.RegisterAttached(
"AutoScroll",
typeof(bool),
typeof(ListBoxControl),
new PropertyMetadata(false));

public static void SetAutoScroll(ListBoxControl element, bool value)

element.SetValue(autoScrollProperty, value);
if (value)

element.SelectionChanged += Element_SelectionChanged;

else

element.SelectionChanged -= Element_SelectionChanged;



public static bool GetAutoScroll(ListBoxControl element)

return (bool)element.GetValue(autoScrollProperty);


private static void Element_SelectionChanged(object sender, SelectionChangedEventArgs e)

var listBox = (ListBoxControl)sender;
listBox.ScrollIntoView(listBox.SelectedItem);





When I use a static True/False value in the xaml, it works fine:

<ListBox ap:ListBox.AutoScroll="True">
...
</ListBox>


But if I data bind to a property in my view model:

<ListBox ap:ListBox.AutoScroll="Binding Path=Settings.EnableAutoScroll">
...
</ListBox>


Then I get the following exception: A 'Binding' cannot be set on the 'SetAutoScroll' property of type 'ListBox'. A 'Binding' can only be set on a DependencyProperty of a DependencyObject.

A 'Binding' cannot be set on the 'SetAutoScroll' property of type 'ListBox'. A 'Binding' can only be set on a DependencyProperty of a DependencyObject.

Is this possible, or am I going to need to derive my own custom list box to accomplish this?

1 Answer
1

Problem in this line typeof(ListBoxControl). You should specify the name of the class where custom attached property seats.

typeof(ListBoxControl)

I would recommend rename class from ListBox to ListBoxExtensions, also, make it static. Then you don't have to use alias ListBoxControl.

Your final code will look like:

public static class ListBoxExtensions

public static readonly DependencyProperty autoScrollProperty =
DependencyProperty.RegisterAttached(
"AutoScroll",
typeof(bool),
typeof(ListBoxExtensions),
new PropertyMetadata(false));

...



Edit:
OK, your code has another problem.

Remove attachment of the listener from setter (SetAutoScroll) and put this logic into dependency property callback.

SetAutoScroll

public static class ListBoxExtensions

public static readonly DependencyProperty autoScrollProperty =
DependencyProperty.RegisterAttached(
"AutoScroll",
typeof(bool),
typeof(ListBoxExtensions),
new PropertyMetadata(false, AutoScrollChangedCallback));

public static void SetAutoScroll(ListBox element, bool value)

element.SetValue(autoScrollProperty, value);


public static bool GetAutoScroll(ListBox element)

return (bool)element.GetValue(autoScrollProperty);


private static void AutoScrollChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)

ListBox control = (ListBox)d;

if ((bool)e.NewValue)

control.SelectionChanged += Element_SelectionChanged;

else

control.SelectionChanged -= Element_SelectionChanged;



private static void Element_SelectionChanged(object sender, SelectionChangedEventArgs e)

var listBox = (ListBox)sender;
listBox.ScrollIntoView(listBox.SelectedItem);




ListBoxControl

System.Windows.Controls.Listbox

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