PrismLibrary/Prism · error · ArgumentNullException

e

Error message

e

What it means

AutoPopulateRegionBehavior.OnViewRegistered handles ViewRegisteredEventArgs from the view registry and throws ArgumentNullException when the event args e is null. It then matches e.RegionName against the behavior's region and adds the registered view into the region.

Solutions

1. Ensure the IRegionViewRegistry always raises ViewRegistered with a non-null ViewRegisteredEventArgs
2. Fix custom registry/event-raising code to construct valid event args
3. If calling OnViewRegistered directly (e.g. in tests), pass a properly constructed ViewRegisteredEventArgs

Example fix

// before
behavior.OnViewRegistered(this, null);

// after
behavior.OnViewRegistered(this, new ViewRegisteredEventArgs { RegionName = "MyRegion", View = viewFactory });

if (args is null) throw new InvalidOperationException("ViewRegisteredEventArgs must be constructed before raising the event");

try { behavior.OnViewRegistered(sender, args); }
catch (ArgumentNullException) { /* registry raised null args; fix event-raising code */ }

When it happens

Trigger: Raising the view-registered callback with null ViewRegisteredEventArgs — i.e. invoking the WeakEvent/subscription path that calls OnViewRegistered(sender, null). Normally only reachable from custom IRegionViewRegistry code or tests firing the event manually.

Common situations: Custom view-registry implementations invoking the handler directly; unit tests simulating view registration with null args; mis-wired weak delegate subscriptions passing null payload.

Related errors

• regionName
• regionTarget
• target
• view
• region

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/6558dd4ed57a2563. Report an issue: GitHub.

Appendix: source

Thrown at src/Maui/Prism.Maui/Navigation/Regions/Behaviors/AutoPopulateRegionBehavior.cs:110

    private void Region_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "Name" && !string.IsNullOrEmpty(Region.Name))
        {
            Region.PropertyChanged -= Region_PropertyChanged;
            StartPopulatingContent();
        }
    }

    /// <summary>
    /// Handler of the event that fires when a new viewtype is registered to the registry.
    /// </summary>
    /// <remarks>Although this is a public method to support Weak Delegates in Silverlight, it should not be called by the user.</remarks>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    public virtual void OnViewRegistered(object sender, ViewRegisteredEventArgs e)
    {
        if (e == null)
            throw new ArgumentNullException(nameof(e));

        if (e.RegionName == Region.Name)
        {
            AddViewIntoRegion((VisualElement)e.GetView(Region.Container()));
        }
    }
}

View on GitHub (pinned to 358118cd64)