lepoco/wpfui · error · InvalidOperationException

The {nameof(viewItem)}.{nameof(viewItem.TargetPageType)} pro

Error message

The {nameof(viewItem)}.{nameof(viewItem.TargetPageType)} property cannot be null.

What it means

Thrown by NavigationView.GetNavigationItemInstance when an INavigationViewItem being navigated to has a null TargetPageType. The navigation system needs a concrete Type to instantiate or resolve the destination page; without it, navigation cannot proceed. This is a configuration error: the item was added without a target page.

Source

Thrown at src/Wpf.Ui/Controls/NavigationView/NavigationView.Navigation.cs:270

        Journal.Add(viewItem.Id);
        _currentIndexInJournal++;

        SetCurrentValue(IsBackEnabledProperty, CanGoBack);

        Debug.WriteLineIf(EnableDebugMessages, $"JOURNAL INDEX {_currentIndexInJournal}");

        if (Journal.Count > 0)
        {
            Debug.WriteLineIf(EnableDebugMessages, $"JOURNAL LAST ELEMENT {Journal[^1]}");
        }
    }

    private object GetNavigationItemInstance(INavigationViewItem viewItem)
    {
        if (viewItem.TargetPageType is null)
        {
            throw new InvalidOperationException(
                $"The {nameof(viewItem)}.{nameof(viewItem.TargetPageType)} property cannot be null."
            );
        }

        if (_serviceProvider is not null)
        {
            return _serviceProvider.GetService(viewItem.TargetPageType)
                ?? throw new InvalidOperationException(
                    $"{nameof(_serviceProvider)}.{nameof(_serviceProvider.GetService)} returned null for type {viewItem.TargetPageType}."
                );
        }

        if (_pageService is not null)
        {
            return _pageService.GetPage(viewItem.TargetPageType)
                ?? throw new InvalidOperationException(
                    $"{nameof(_pageService)}.{nameof(_pageService.GetPage)} returned null for type {viewItem.TargetPageType}."
                );

View on GitHub (pinned to ffebacd610)

Solutions

Set TargetPageType on every navigable NavigationViewItem (in XAML: TargetPageType="{x:Type pages:MyPage}").
For non-navigating items (headers, separators), make them non-selectable so they cannot trigger navigation.
If binding from a view-model, ensure the model exposes a non-null TargetPageType.
Validate the item before calling Navigate.

Example fix

<!-- before -->
<ui:NavigationViewItem Content="Settings"/> <!-- throws when selected -->

<!-- after -->
<ui:NavigationViewItem Content="Settings" TargetPageType="{x:Type pages:SettingsPage}"/>

Defensive patterns

Strategy: validation

Validate before calling

if (viewItem.TargetPageType is null)
    throw new InvalidOperationException(
        $"NavigationViewItem '{viewItem.Content}' has no TargetPageType.");

Type guard

static bool IsNavigable(INavigationViewItem i) => i.TargetPageType is not null;

Prevention

Set TargetPageType on every selectable NavigationViewItem.
Make header/separator items non-selectable.
Validate the item in DEBUG before calling Navigate.

When it happens

Trigger: Selecting/clicking a NavigationViewItem whose TargetPageType is null; programmatic Navigate(...) with an item whose TargetPageType was never set; XAML <ui:NavigationViewItem> without a TargetPageType attribute and no other page-resolution mechanism.

Common situations: Adding a header-only or separator NavigationViewItem that is selectable but has no page; binding NavigationViewItem instances from a view-model that omits the page type; XAML where the TargetPageType attribute was removed during refactoring.

Related errors

AI-assisted analysis of lepoco/wpfui@ffebacd610 (2026-08-13). Data as JSON: /api/errors/9af97ddbb0c2ad03. Report an issue: GitHub.