Yup - get TCPView from SysInternals, find the application and the connection, and close it. Failing that, restarting the application, unless the specific app has a way of doing this. Most apps won't.

How to forcibly disconnect a application listening on a port
I have encountered something very strange, simple WPF application

&lt;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"&gt;
    &lt;Grid&gt;
        &lt;ListBox ItemsSource="{Binding Path=Strings}" SelectionMode="Single"/&gt;
    &lt;/Grid&gt;
&lt;/Window&gt;


with code behind

public class ViewModel
{
    public List&lt;string&gt; Strings { get; set; }

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

/// &lt;summary&gt;
/// Interaction logic for MainWindow.xaml
/// &lt;/summary&gt;
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.
