ListBox is selecting many items even in SelectionMode="Single"

Question

I have encountered something very strange, simple WPF application

<Window x:Class="ListBoxSelection.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">
    <Grid>
        <ListBox ItemsSource="{Binding Path=Strings}" SelectionMode="Single"/>
    </Grid>
</Window>

with code behind

public class ViewModel
{
    public List<string> Strings { get; set; }

    public ViewModel ()
    {
        Strings = new List<string> ();
        Strings.Add ("A");
        // add many items ...
        Strings.Add ("A");
    }
}

/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
    public MainWindow ()
    {
        InitializeComponent ();

        DataContext = new ViewModel ();
    }
}

and when I click on a single item,

if I continue clicking items, they just aggregate. Clicking an already selected item does nothing. Scratching my head, I have databound lists to ListBoxes before, and have never seen this before. Running Win7 (64), VS2010, behaviour presents with .Net 3.5, .Net 3.5 Client Profile, .Net 4, and .Net 4 Client Profile.

Arg, I should mention I am expecting normal, default, single-select behaviour.

Answer 1

Dan Bryant got most of the answer in his comment.

What's going on here is string interning. When you create a bunch of strings with the same value, .Net saves on memory usage by having all references to the same string value actually refer to the same string object. (See this, for instance, for details.)

I don't really know why the ListBox behaves exactly the way it does, which is that the first time you select any item in the list, it selects both that item and the first item in the list. But it doesn't unselect when you click on a new item because checks to see if the SelectedItem is different from the item you just clicked on, and it isn't.

I got exactly the same behavior by binding a ListBox to a collection of test objects:

public class TestObject
{
    public override string ToString()
    {
        return GetHashCode().ToString();
    }
}

In MainWindow.xaml:

<ListBox x:Name="MyListBox" ItemsSource={Binding}"/>

In MainWindow.xaml.cs:

ObservableCollection<TestObject> test = new ObservableCollection<TestObject>();
TestObject t = new TestObject();
test.Add(t);
test.Add(t);
test.Add(t);
test.Add(t);
test.Add(t);
test.Add(t);
MyListBox.DataContext = test;