WinUI.TableView with C# Markup

You’re building a nice Uno Platform app with C# Markup, you drop in WinUI.TableView, wire up ItemsSource the usual way — and the app crashes.

InvalidOperationException: Setting this property directly is not allowed. Use TableView.ItemsSource instead.

Confusing, right? Your code looks correct. The binding syntax is fine. So what’s going on?

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The Problem

C# Markup gives you a clean fluent API:

new TableView()
    .ItemsSource(() => ViewModel.People);

Under the hood though, C# Markup resolves ItemsSource through ItemsControl — because that’s where the extension methods are defined. So it ends up setting:

ItemsControl.ItemsSourceProperty

That’s normally fine for standard controls. But WinUI.TableView hides the base property:

public new object? ItemsSource { get; set; }

We do this to support features like sorting and filtering, which require intercepting every assignment. Along with that, we register our own dependency property:

public new static DependencyProperty ItemsSourceProperty

So you end up with two completely separate DPs living side by side:

• ItemsControl.ItemsSourceProperty
• TableView.ItemsSourceProperty

C# Markup targets the first one. TableView guards it and throws. That’s your crash.

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Why This Happens

Neither C# Markup nor TableView is doing something wrong here.

This is just property hiding in .NET — when you declare public new, you’re not overriding anything. You’re creating a brand new member that has no relationship to the base class version. C# Markup has no way of knowing that TableView wants a different DP, so it falls back to the one it knows about.

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The Fix

Because TableView.ItemsSource is of type object, the built-in C# Markup extensions don’t apply. We need custom ones that target TableView.ItemsSourceProperty directly, covering all the common usage patterns:

public static class TableViewMarkupExtensions
{
    // 1️⃣ Direct assignment
    public static T ItemsSource<T>(this T element, object itemsSource) where T : TableView
    {
        element.ItemsSource = itemsSource;
        return element;
    }

    // 2️⃣ Binding via DependencyPropertyBuilder
    public static T ItemsSource<T>(this T element, Action<IDependencyPropertyBuilder<object>> configureProperty) where T : TableView
    {
        var instance = DependencyPropertyBuilder<object>.Instance;
        configureProperty(instance);
        instance.SetBinding(element, ItemsControl.ItemsSourceProperty, nameof(TableView.ItemsSource));
        return element;
    }

    // 3️⃣ Strongly-typed binding
    public static T ItemsSource<T, TSource>(this T element, Func<TSource> propertyBinding, [CallerArgumentExpression(nameof(propertyBinding))] string? propertyBindingExpression = null) where T : TableView
    {
        return ItemsSource(element, delegate (IDependencyPropertyBuilder<object> _)
        {
            _.Binding(propertyBinding, propertyBindingExpression);
        });
    }

    // 4️⃣ Strongly-typed binding with conversion
    public static T ItemsSource<T, TSource>(this T element, Func<TSource> propertyBinding, Func<TSource, object> convertDelegate, [CallerArgumentExpression("propertyBinding")] string? propertyBindingExpression = null) where T : TableView
    {
        return ItemsSource(element, delegate (IDependencyPropertyBuilder<object> _)
        {
            _.Binding(propertyBinding, propertyBindingExpression).Convert(convertDelegate);
        });
    }
}

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Now It Works

With these extensions in place, both styles work exactly as you’d expect:

new TableView()
    .ItemsSource(People);

new TableView()
    .ItemsSource(() => vm.People);

The binding now correctly targets TableView.ItemsSourceProperty and the crash is gone.

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