x:Bind to function with null reference in path throws NullReferenceException in Debug mode

[bjorn-malmo]

Describe the bug

When binding to a function, where part of the path is null and supplying a parameter that is related to a UI element, a NullReferenceException is thrown.
It does not respect the FallbackValue.

It only happens in Debug mode, and not in Release mode, but it makes it difficult to actually use this feature.

Having a static parameter value for instance works (returning null, or the FallbackValue if configured).

So, this line works:
<TextBlock Text="{x:Bind NullInstance.MyFunction(20), FallbackValue='null'}"/>

but this does not:
<TextBlock Text="{x:Bind NullInstance.MyFunction(RootElement.ActualWidth), FallbackValue='null'}"/>

Steps to reproduce the bug

Create a WinUI 3 project, add a class with a function:

    public class TestClass
    {
        public string MyFunction(double width)
        {
            return width.ToString();
        }
    }

In the code behind for the main window, add:

        public TestClass Instance { get; } = new TestClass();
        public TestClass NullInstance { get; } = null;

and in the Xaml for the main window, add:

    <Grid x:Name="RootElement">
        <StackPanel>
            <TextBlock Text="{x:Bind NullInstance.MyFunction(20), FallbackValue='null'}"/>
            <TextBlock Text="{x:Bind Instance.MyFunction(RootElement.ActualWidth), FallbackValue='null'}"/>
        </StackPanel>
    </Grid>

This example will work, but changing the Xaml to bind to NullInstance throws.

Expected behavior

Expected the FallbackValue to be returned, or null if none defined.

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.6.6: 1.6.250228001

Windows version

Windows 11 (24H2): Build 26100

Additional context

This is reproducible in App SDK 1.7 as well

[Zakariathr22]

The issue is not with FallbackValue itself. If you run the app in Release mode, it will work as expected, and the FallbackValue will be returned.

I suspect this behavior is caused by the XAMLC or the Debugger, or both. You can test this by switching to Release mode and verifying if the issue disappears.

[bjorn-malmo]

Thank you! It works exactly as you said.

Too bad that it makes it difficult to use since the debugging experience is broken, but I will not pursue this further :)

[Zakariathr22]

I suggest reopening the issue with a different title, focusing on the difference in behavior between Debug and Release modes.

[bjorn-malmo]

I have edited the original issue as suggested

[snigdha011997]

Hi @bjorn-malmo , can you help us by providing minimal repro project?