Accessing the ScrollViewer of a ListBox from C#

Question

I'd like to change the properties of a ScrollViewer of a ListBox from C#.

I found this question here on Stackoverflow. I took the accepted answer's advice and exposed the ScrollViewer as a property of a subclass. However, this doesn't appear to be working in an example shown below. Some of the comments in that question also state that this technique didn't work.

XAML:

<Window x:Class="StackoverflowListBoxScrollViewer.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">

</Window>

C#:

using System;
using System.Windows;
using System.Windows.Controls;

namespace StackoverflowListBoxScrollViewer
{
    public class MyListBox : ListBox
    {
        public ScrollViewer ScrollViewer
        { get { return (ScrollViewer)GetTemplateChild("ScrollViewer"); } }
    }

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            var myListBox = new MyListBox();

            Content = myListBox;

            myListBox.Items.Add(new Button() { Content = "abc" });
            myListBox.Items.Add(new Button() { Content = "abc" });
            myListBox.Items.Add(new Button() { Content = "abc" });
            myListBox.Items.Add(new Button() { Content = "abc" });
            myListBox.Items.Add(new Button() { Content = "abc" });

            var button = new Button() { Content = "Check ScrollViewer" };
            button.Click += (s, e) =>
                {
                    if (myListBox.ScrollViewer == null)
                        Console.WriteLine("null");
                };
            myListBox.Items.Add(button);
        }
    }
}

When I click the "Check ScrollViewer" button, it prints "null". I.e., the ScrollViewer wasn't retrieved.

How do I get to that darn ScrollViewer? :-)

Answer 1

you can try this little helper function

usage

var scrollViewer = GetDescendantByType(yourListBox, typeof(ScrollViewer)) as ScrollViewer;

helper function

public static Visual GetDescendantByType(Visual element, Type type)
{
  if (element == null) {
    return null;
  }
  if (element.GetType() == type) {
    return element;
  }
  Visual foundElement = null;
  if (element is FrameworkElement) {
    (element as FrameworkElement).ApplyTemplate();
  }
  for (int i = 0; i < VisualTreeHelper.GetChildrenCount(element); i++) {
    Visual visual = VisualTreeHelper.GetChild(element, i) as Visual;
    foundElement = GetDescendantByType(visual, type);
    if (foundElement != null) {
      break;
    }
  }
  return foundElement;
}

Hope this helps

Answer 2

If you will use standard ListBox, so you can change yours getter to this one:

public class MyListBox : ListBox
{
    public ScrollViewer ScrollViewer
    {
        get 
        {
            Border border = (Border)VisualTreeHelper.GetChild(this, 0);

            return (ScrollViewer)VisualTreeHelper.GetChild(border, 0);
        }
    }
}

Answer 3

I've modified the great answer of @punker76 to create an extension method for Visual and provide explicit return type:

   public static class Extensions
   {
      public static T GetDescendantByType<T>(this Visual element) where T:class
      {
         if (element == null)
         {
            return default(T);
         }
         if (element.GetType() == typeof(T))
         {
            return element as T;
         }
         T foundElement = null;
         if (element is FrameworkElement)
         {
            (element as FrameworkElement).ApplyTemplate();
         }
         for (var i = 0; i < VisualTreeHelper.GetChildrenCount(element); i++)
         {
            var visual = VisualTreeHelper.GetChild(element, i) as Visual;
            foundElement = visual.GetDescendantByType<T>();
            if (foundElement != null)
            {
               break;
            }
         }
         return foundElement;
      }

   }

You can now call it by SomeVisual.GetDescendantByType and it returns either already a correct typed ScrollViewer or null (which is default(T))