WinUI3 ListView Selected Items

I'm using a WinUI3 ListView like this to load a list of files:

<ListView ItemTemplate="{StaticResource Template2}"
 ItemsSource="{x:Bind Files,Mode=OneWay}"  
IsItemClickEnabled="True" x:Name="List2" SelectionMode="Multiple" />
 <DataTemplate x:Key="Template2" x:DataType="local:FileItem">
        <Grid>
         .... TextBlocks that bind to properties in FileItem
         </Grid>
</DataTemplate>

Now, how do I configure my DataTemplate in order for some (or all) of the items to be selected by default?

Is there a special value in the data template I should use?

You can use the ListViewItem in your ItemTemplate.

Let's say your FileItem looks like this:

public partial class FileItem : ObservableObject
{
    [ObservableProperty]
    private string _name = string.Empty;

    [ObservableProperty]
    private bool _isSelected;
}

and your ViewModel:

public partial class  MainPageViewModel : ObservableObject
{
    [ObservableProperty]
    private ObservableCollection<FileItem> _files = new()
    {
        new FileItem { Name = "File1" },
        new FileItem { Name = "File2", IsSelected = true, },
        new FileItem { Name = "File3"},
    };
}

and in XAML:

<Page.Resources>
    <DataTemplate
        x:Key="Template2"
        x:DataType="local:FileItem">
        <ListViewItem IsSelected="{x:Bind IsSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
            <TextBlock Text="{x:Bind Name, Mode=OneWay}" />
        </ListViewItem>
    </DataTemplate>
</Page.Resources>
<ListView
    Grid.Row="1"
    IsItemClickEnabled="True"
    ItemTemplate="{StaticResource Template2}"
    ItemsSource="{x:Bind ViewModel.Files, Mode=OneWay}"
    SelectionMode="Multiple" />

But unfortunately, this won't work because the ListViewItem resets its IsSelected property to false when it's loaded. I'm not sure if it's a bug or it's by design.

As a workaround, you can do the binding after each ListViewItem is loaded:

<Page.Resources>
    <DataTemplate
        x:Key="Template2"
        x:DataType="local:FileItem">
        <ListViewItem Loaded="ListViewItem_Loaded">
            <TextBlock Text="{x:Bind Name, Mode=OneWay}" />
        </ListViewItem>
    </DataTemplate>
</Page.Resources>

then in code-behind:

private void ListViewItem_Loaded(object sender, RoutedEventArgs e)
{
    if (sender is not ListViewItem listViewItem)
    {
        return;
    }

    listViewItem.SetBinding(
        ListViewItem.IsSelectedProperty,
        new Binding
        {
            Path = new PropertyPath("IsSelected"),
            Mode = BindingMode.TwoWay,
            Source = listViewItem.DataContext,
            UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged,
        });
}

BTW, I'm using the CommunityToolkit.Mvvm NuGet package for the ViewModels but I hope this gives you an idea how to fix your issue.