{"record":{"id":"e2631ff3298e6739","repo":"PrismLibrary/Prism","slug":"resources-regionbehaviorattachcannotbecallwithnullregion","errorCode":null,"errorMessage":"Resources.RegionBehaviorAttachCannotBeCallWithNullRegion","messagePattern":"Resources\\.RegionBehaviorAttachCannotBeCallWithNullRegion","errorType":"exception","errorClass":"InvalidOperationException","httpStatus":null,"severity":"error","filePath":"src/Prism.Core/Navigation/Regions/RegionBehavior.cs","lineNumber":45,"sourceCode":"                }\n\n                this.region = value;\n            }\n        }\n\n        /// <summary>\n        /// Returns <see langword=\"true\"/> if the behavior is attached to a region, <see langword=\"false\"/> otherwise.\n        /// </summary>\n        public bool IsAttached { get; private set; }\n\n        /// <summary>\n        /// Attaches the behavior to the region.\n        /// </summary>\n        public void Attach()\n        {\n            if (this.region == null)\n            {\n                throw new InvalidOperationException(Resources.RegionBehaviorAttachCannotBeCallWithNullRegion);\n            }\n\n            IsAttached = true;\n            OnAttach();\n        }\n\n        /// <summary>\n        /// Override this method to perform the logic after the behavior has been attached.\n        /// </summary>\n        protected abstract void OnAttach();\n    }\n}\n","sourceCodeStart":27,"sourceCodeEnd":58,"githubUrl":"https://github.com/PrismLibrary/Prism/blob/358118cd640d9a22ff8cf21c8ad197fa038b7990/src/Prism.Core/Navigation/Regions/RegionBehavior.cs#L27-L58","documentation":"InvalidOperationException in RegionBehavior.Attach: Attach() was called before the Region property was assigned a region. A behavior cannot attach to nothing — it needs the region instance to hook into — so attaching while this.region is null throws with the RegionBehaviorAttachCannotBeCallWithNullRegion resource message. Ensure Region is set (typically via RegionBehaviorCollection.Add) before calling Attach.","triggerScenarios":"Calling behavior.Attach() directly without first assigning behavior.Region = someRegion.","commonSituations":"Manually wiring behaviors in a custom IRegionAdapter and forgetting the Region assignment; constructing a behavior in code and attaching immediately.","solutions":["Set the Region property before calling Attach()","Prefer adding the behavior to the region's RegionBehaviorCollection, which sets Region and calls Attach","Guard: if (behavior.Region == null) behavior.Region = region;"],"exampleFix":"// before\nvar b = new MyBehavior();\nb.Attach(); // Region never set\n// after\nvar b = new MyBehavior { Region = region };\nb.Attach();","handlingStrategy":"validation","validationCode":"if (behavior.Region == null) behavior.Region = region; behavior.Attach();","typeGuard":"bool ReadyToAttach(IRegionBehavior b) => b.Region != null && !b.IsAttached;","tryCatchPattern":"try { behavior.Attach(); } catch (InvalidOperationException) { /* set Region first */ }","preventionTips":["Assign Region before Attach","Prefer region.Behaviors.Add(key, behavior) over manual Attach"],"tags":["prism","regions","invalid-operation","null"],"backgroundTag":"missing-required-argument","analyzedSha":"358118cd640d9a22ff8cf21c8ad197fa038b7990","analyzedAt":"2026-09-15T15:00:31.079Z","contentChangedAt":"2026-09-15T15:00:31.079Z","schemaVersion":2},"datasetVersion":"2026-09-16T04:17:20.429Z"}