WPF unselect ListBox item in MVVM

Question

I added ListBox to my XAML file which look like this:

<ListBox
    SelectedIndex="{Binding SelectedIndex}"
    ItemsSource="{Binding  Answers}">

    ...

</ListBox>

My ViewModel class have these properties:

private int? selectedIndex;
public int? SelectedIndex
{
    get => selectedIndex;
    set
    {
        selectedIndex = value;
        RaisePropertyChanged(nameof(SelectedIndex));
    }
}

private ObservableCollection<string> answers;
public ObservableCollection<string> Answers
{
    get => answers;
    private set
    {
        answers = value;
        RaisePropertyChanged(nameof(Answers));
    }
}

Now when I click on the Item that is already selected I want to unselect this Item (so I think SelectedIndex = null will do the job). How I can do it? I try to find a solution but without any success.

Is there any command which I could execute every time the ListBox item is clicked? This command as a parameter must pass the index of clicked item. If there is a possibility like this it will be enough for me if You just show me how to create this command and pass the Item index as a parameter.

Answer 1

You can set up a PreviewMouseLeftButtonDown (or similar) event handler

<ListBox ...>
    <ListBox.ItemContainerStyle>
        <Style TargetType="ListBoxItem">
            <EventSetter Event="PreviewMouseLeftButtonDown"
                         Handler="ListBoxItemPreviewMouseLeftButtonDown"/>
        </Style>
    </ListBox.ItemContainerStyle>
</ListBox>

that does just this:

private void ListBoxItemPreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    if (sender is ListBoxItem listBoxItem && listBoxItem.IsSelected)
    {
        listBoxItem.Dispatcher.InvokeAsync(() => listBoxItem.IsSelected = false);
    }
}

Answer 2

I added ListBox to my XAML file which look like this:

<ListBox
    SelectedIndex="{Binding SelectedIndex}"
    ItemsSource="{Binding  Answers}">

    ...

</ListBox>

My ViewModel class have these properties:

private int? selectedIndex;
public int? SelectedIndex
{
    get => selectedIndex;
    set
    {
        selectedIndex = value;
        RaisePropertyChanged(nameof(SelectedIndex));
    }
}

private ObservableCollection<string> answers;
public ObservableCollection<string> Answers
{
    get => answers;
    private set
    {
        answers = value;
        RaisePropertyChanged(nameof(Answers));
    }
}

Now when I click on the Item that is already selected I want to unselect this Item (so I think SelectedIndex = null will do the job). How I can do it? I try to find a solution but without any success.

Is there any command which I could execute every time the ListBox item is clicked? This command as a parameter must pass the index of clicked item. If there is a possibility like this it will be enough for me if You just show me how to create this command and pass the Item index as a parameter.