Compiled Binding to Array.Count provides no result

[omghb]

Description

Compiled Binding to Array.Count provides no result. Building works without any error or warning.

Last version that worked well: Xamarin Forms 5.0.

We run into this issue during migration from Xamarin Forms 5.0 to MAUI 7.0. In Xamarin Forms this worked well.

Steps to Reproduce

1. Create a new empty MAUI app
2. Replace content of MainPage.xaml with:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:MauiApp2"
             x:Class="MauiApp2.MainPage" x:DataType="local:MainViewModel">

    <VerticalStackLayout>
        <Label Text="List.Count:"/>
        <Label Text="{Binding List.Count}"/>

        <Label Text="ListCount:"/>
        <Label Text="{Binding ListCount}"/>
    </VerticalStackLayout>
</ContentPage>

3. Replace MainPage.xaml.cs with

namespace MauiApp2;

public partial class MainPage : ContentPage
{
	public MainPage()
	{
		InitializeComponent();
		BindingContext = new MainViewModel();
	}
}

public class MainViewModel
{
    private readonly string[] list = new[] { "Bill", "Steve", "John" };

    public IReadOnlyList<string> List => list;

    public int ListCount => list.Length;
}

Note: You might need to adapt the namespace.

4. Run application.

• See that first Binding results in an empty value -> BUG
• Second Binding results in the value 3 -> correct.

Link to public reproduction project repository

Version with bug

7.0 (current)

Last version that worked well

Unknown/Other

Affected platforms

iOS, Android, Windows

Affected platform versions

All

Did you find any workaround?

No response

Relevant log output

No response

We've added this issue to our backlog, and we will work to address it as time and resources allow. If you have any additional information or questions about this issue, please leave a comment. For additional info about issue management, please read our Triage Process.

[mattleibow]

@StephaneDelcroix thoughts?

[StephaneDelcroix]

    public int ListCount => list.Length;

🧐

what would List.Count return ?

[omghb]

It returns 3. That's the tricky part 😉.

An Array has not a visible Count property. The visible property is the Length property.

However, an Array implements the interfaces ICollection, ICollection<T> and IReadOnlyCollection<T>. All of them declare a property Count. So the Array supports the Count property when used via interface.

We expose Collections often via IReadOnlyList<T> within a ViewModel and Bind to them. In Xamarin it was possible to Bind to the Count property. In MAUI it seems that this is not working anymore.

We could rewrite the example also like this:

public int ListCount => List.Count;

[Zhanglirong-Winnie]

Verified this issue with Visual Studio Enterprise 17.7.0 Preview 2.0. Can repro on android and windows platform with above code.

[StephaneDelcroix]

My current tests shows that it actually works with XamlC on (and compiled bindings) like in Release but fails in DEBUG. and it's not beer-o-clock yet

[StephaneDelcroix]

this probably never worked on XF, the difference being that maui disable Xaml compilation on debug builds

[omghb]

It works with XF 5 for a released and published App (in Debug and Release mode).

We run into multiple bugs because of this issue after migration to MAUI. And we had to implement workarounds for those bindings so that the App works with MAUI again.