{"record":{"id":"ee7439424354cc1e","repo":"PrismLibrary/Prism","slug":"argumentnullexception-for-view-regionmanager","errorCode":null,"errorMessage":"ArgumentNullException for 'view'","messagePattern":"ArgumentNullException for 'view'","errorType":"exception","errorClass":"ArgumentNullException","httpStatus":null,"severity":"error","filePath":"src/Maui/Prism.Maui/Navigation/Regions/Xaml/RegionManager.cs","lineNumber":121,"sourceCode":"        if (regionTarget == null)\n            throw new ArgumentNullException(nameof(regionTarget));\n\n        return regionTarget.GetValue(RegionNameProperty) as string;\n    }\n\n    /// <summary>\n    /// Returns an <see cref=\"ObservableObject{T}\"/> wrapper that can hold an <see cref=\"IRegion\"/>. Using this wrapper\n    /// you can detect when an <see cref=\"IRegion\"/> has been created by the <see cref=\"RegionAdapterBase{T}\"/>.\n    ///\n    /// If the <see cref=\"ObservableObject{T}\"/> wrapper does not yet exist, a new wrapper will be created. When the region\n    /// gets created and assigned to the wrapper, you can use the <see cref=\"ObservableObject{T}\"/> event\n    /// to get notified of that change.\n    /// </summary>\n    /// <param name=\"view\">The view that will host the region. </param>\n    /// <returns>Wrapper that can hold an <see cref=\"IRegion\"/> value and can notify when the <see cref=\"IRegion\"/> value changes. </returns>\n    public static ObservableObject<IRegion> GetObservableRegion(VisualElement view)\n    {\n        if (view is null) throw new ArgumentNullException(nameof(view));\n\n        if (view.GetValue(ObservableRegionProperty) is not ObservableObject<IRegion> regionWrapper)\n        {\n            regionWrapper = new ObservableObject<IRegion>();\n            view.SetValue(ObservableRegionProperty, regionWrapper);\n        }\n\n        return regionWrapper;\n    }\n\n    private static void OnSetRegionNameCallback(BindableObject bindable, object oldValue, object newValue)\n    {\n        if (DesignMode.IsDesignModeEnabled || newValue is null || bindable is not VisualElement view)\n            return;\n\n        CreateRegion(view);\n    }\n","sourceCodeStart":103,"sourceCodeEnd":139,"githubUrl":"https://github.com/PrismLibrary/Prism/blob/358118cd640d9a22ff8cf21c8ad197fa038b7990/src/Maui/Prism.Maui/Navigation/Regions/Xaml/RegionManager.cs#L103-L139","documentation":"RegionManager.GetObservableRegion returns (or lazily creates) an ObservableObject<IRegion> wrapper stored on the view, allowing callers to observe when the region changes. It throws ArgumentNullException when the view is null because no wrapper can be attached to a null element.","triggerScenarios":"Calling RegionManager.GetObservableRegion(null).","commonSituations":"Subscribing to region changes in a page whose content element was not yet created; passing a ViewModel instead of the view.","solutions":["Pass the view (VisualElement) that hosts the region, not a ViewModel or null.","Ensure the view's visual tree element exists before obtaining the observable wrapper."],"exampleFix":"// before\nRegionManager.GetObservableRegion(null).Value = region;\n// after\nif (view is not null)\n    RegionManager.GetObservableRegion(view).Value = region;","handlingStrategy":"type-guard","validationCode":"if (view is null) throw new InvalidOperationException(\"View must exist before obtaining observable region\");","typeGuard":"if (view is VisualElement ve) { var wrapper = RegionManager.GetObservableRegion(ve); }","tryCatchPattern":"try { RegionManager.GetObservableRegion(view); }\ncatch (ArgumentNullException) { logger.LogWarning(\"GetObservableRegion called with null view\"); }","preventionTips":["Pass the VisualElement view, never a ViewModel.","Call after InitializeComponent so the element exists."],"tags":["prism","maui","regions","argument-null","observable"],"backgroundTag":"null-argument","analyzedSha":"358118cd640d9a22ff8cf21c8ad197fa038b7990","analyzedAt":"2026-09-15T15:00:31.079Z","contentChangedAt":"2026-09-15T15:00:31.079Z","schemaVersion":2},"datasetVersion":"2026-09-23T08:17:48.524Z"}