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Angular’s NG0912 warning means two component definitions generated the same component ID from their metadata. Read the two class names in the warning, inspect both components, then give them distinct metadata—usually by changing one selector or adding a host attribute. Angular documents both remedies in its NG0912 guide.
What NG0912 means
Angular derives a component ID from component metadata, including selectors, host bindings, class property names, and view or content queries. If two component definitions produce the same ID, Angular reports NG0912. A shared selector is a common cause, but it is not the only metadata that can contribute to a collision.
In Angular’s documented example, two components share the my-component selector but have different templates. Different template text alone does not ensure distinct component IDs: the ID is based on component metadata, not simply the class name or template.
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Angular uses component IDs for additional annotations on DOM nodes related to style encapsulation and for hydration, which restores application state after server-side rendering. In emulated style encapsulation, Angular adds an attribute to elements in a component’s template and uses that attribute in component CSS selectors. The ID supports these framework behaviors, so treat NG0912 as a collision to resolve rather than a warning to suppress. See Angular’s styling guide for how encapsulation scopes component styles.
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How to fix NG0912
- Read the warning and locate both components. The message identifies the two component classes involved. Find their definitions and compare their metadata.
- Give the components distinct selectors when possible. If they are meant to be different components in templates, change one selector so it is unique. This is the most direct documented fix.
- Add a host attribute if the selectors must stay the same. Angular documents adding an extra host property, such as
some-binding, to make one component’s metadata distinct.
These two options address the metadata collision directly. Choose a unique selector when the components should be addressed differently in templates; choose an extra host attribute when retaining identical selectors is necessary. Avoid changing view encapsulation as a workaround: Angular’s NG0912 guidance recommends changing the selector or adding a host attribute.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where to look when the components seem unrelated
The warning may identify classes that do not appear to overlap in ordinary application code. Check whether one of the definitions comes from a test mock, a library secondary entry point, or a selector-less component created dynamically. Historical Angular issue reports describe these contexts: issue #51850 involved a library setup on Angular 16.2.6, and issue #53282 involved selector-less dynamically displayed components on Angular 17.0.5 and was later closed as a duplicate. These reports suggest places to investigate; they do not establish a general framework defect or current-release behavior.
Once you find both definitions, compare their selectors and other metadata rather than assuming the warning is caused by the templates. Then apply one of Angular’s documented metadata changes.
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