lepoco/wpfui · error · InvalidOperationException

Unable to get or create instance of {viewItem.TargetPageType

Error message

Unable to get or create instance of {viewItem.TargetPageType} from cache.

What it means

Thrown when neither IServiceProvider nor INavigationViewPageProvider is configured, and the cache's Remember factory (which falls through to GetPageInstanceFromCache) also returns null. This is the last-resort resolution path: with no DI and no page provider, the cache must produce an instance, and a null result means even the cache factory could not create one — typically because Activator/CreateInstance failed or returned null.

Source

Thrown at src/Wpf.Ui/Controls/NavigationView/NavigationView.Navigation.cs:296

                ?? 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}."
                );
        }

        return _cache.Remember(
                viewItem.TargetPageType,
                viewItem.NavigationCacheMode,
                ComputeCachedNavigationInstance
            )
            ?? throw new InvalidOperationException(
                $"Unable to get or create instance of {viewItem.TargetPageType} from cache."
            );

        object? ComputeCachedNavigationInstance() => GetPageInstanceFromCache(viewItem.TargetPageType);
    }

    private object? GetPageInstanceFromCache(Type? targetPageType)
    {
        if (targetPageType is null)
        {
            return default;
        }

        if (_serviceProvider is not null)
        {
            System.Diagnostics.Debug.WriteLine(
                $"Getting {targetPageType} from cache using IServiceProvider."
            );

View on GitHub (pinned to ffebacd610)

Solutions

Ensure TargetPageType is a concrete class with a public parameterless constructor.
If the page needs dependencies, configure an IServiceProvider (so error 35's path is used instead) or an INavigationViewPageProvider.
If the type legitimately cannot be default-constructed, supply a page provider that builds it.
Check the inner exception / debug output from NavigationViewActivator to see why instantiation failed.

Example fix

// before
public abstract class SettingsPage : Page {} // throws on navigate

// after
public class SettingsPage : Page
{
    public SettingsPage() { InitializeComponent(); }
}

Defensive patterns

Strategy: validation

Validate before calling

var t = viewItem.TargetPageType;
if (t is null || t.IsAbstract || t.IsInterface || t.GetConstructor(Type.EmptyTypes) is null)
    throw new InvalidOperationException(
        $"{t} must be a concrete type with a public parameterless constructor, or supply a page provider.");

Type guard

static bool IsInstantiable(Type? t) =>
    t is not null && !t.IsAbstract && !t.IsInterface &&
    t.GetConstructor(Type.EmptyTypes) is not null;

Prevention

Point TargetPageType only at concrete, parameterless-constructible classes.
If the page has dependencies, supply an IServiceProvider or page provider.
Check NavigationViewActivator debug output when instantiation fails.

When it happens

Trigger: Navigating to a page with no service provider and no page service, where the cache's ComputeCachedNavigationInstance (-> GetPageInstanceFromCache) returns null — e.g. the target type is abstract, an interface, lacks a parameterless constructor, or NavigationViewActivator.CreateInstance returned null.

Common situations: TargetPageType points to an interface or abstract class; the page type has no public parameterless constructor; the type is a generic open definition; an exception inside the activator was swallowed and returned null.

Related errors

AI-assisted analysis of lepoco/wpfui@ffebacd610 (2026-08-13). Data as JSON: /api/errors/083130618228e2ea. Report an issue: GitHub.