[Desktop] Unexpected null value passed to ItemTemplateSelector when used on Canvas

[kucint]

Current behavior

Create canvas that displays two types of objects ItemA and ItemB and define a ItemTemplateSelector with two DataTemplates for both item types:

<Page x:Class="UnoTemplateSelectorOnCanvasApp.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:local="using:UnoTemplateSelectorOnCanvasApp"
      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <Page.Resources>

        <DataTemplate x:Key="ItemTemplateA">
            <Border Background="LightGreen" Width="50" Height="50"/>
        </DataTemplate>

        <DataTemplate x:Key="ItemTemplateB">
            <Border Background="LightBlue" Width="50" Height="50"/>
        </DataTemplate>

        <local:ItemTemplateSelector x:Key="ItemTemplateSelector"
                                    TemplateA="{StaticResource ItemTemplateA}"
                                    TemplateB="{StaticResource ItemTemplateB}"/>
    </Page.Resources>
    
    <ItemsControl x:Name="ItemPanel" ItemsSource="{x:Bind Items, Mode=OneWay}" ItemTemplateSelector="{StaticResource ItemTemplateSelector}" >
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <Canvas Background="LightYellow"/>
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
    </ItemsControl>
</Page>

This works fine on WindowsSDK Framework:

But on Desktop Framework the ItemTemplateSelector unexpectedly gets null as an item reference where only ItemA and ItemB are expected.

internal sealed partial class ItemTemplateSelector : DataTemplateSelector
{
    public DataTemplate? TemplateA { get; set; }
    public DataTemplate? TemplateB { get; set; }

    protected override DataTemplate? SelectTemplateCore(object item, DependencyObject container) => item switch
    {
        ItemA => TemplateA,
        ItemB => TemplateB,

        // this should never happen: only ItemA and ItemB are expected to be passed to this selector
        // -> On WindowsSDK Framework: all is fine, null never happens
        // -> on Desktop Framework: null is passed to this selector (bug)
        //null => null, // uncomment this line for work-around

        // this should never happen
        _ => throw new InvalidOperationException($"Unrecognized view model {item.GetType()}"),
    };
}

Expected behavior

null shall never be passed to the ItemTemplateSelector

How to reproduce it (as minimally and precisely as possible)

STEPS TO REPRODUCE:

• download the app
• compile and run

MINIMAL REPRO PROJECT: UnoTemplateSelectorOnCanvasApp.zip

Workaround

handle null value in ItemTemplateSelector:
null => null,

Works on UWP/WinUI

Yes

Environment

No response

NuGet package version(s)

"Uno.Sdk": "5.5.32"

Affected platforms

Skia (WPF)

IDE

Visual Studio 2022

IDE version

No response

Relevant plugins

No response

Anything else we need to know?

No response

[jeromelaban]

Thanks for the report.

This is a known behavior at this time, and also a good practice to react to null being passed as parameter, as this is a valid use case from items sources.

[kucint]

@jeromelaban
ok. thanks for info.

[MartinZikmund]

@kucint you can safely return null in case of null parameter