angular-cn/aio/content/errors/NG2009.md

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@name Invalid Shadow DOM selector
@category compiler
@shortDescription Component selector does not match shadow DOM requirements
@description
The selector of a component using `ViewEncapsulation.ShadowDom` doesn't match the custom element tag name requirements.
In order for a tag name to be considered a valid custom element name, it has to:
* Be in lower case.
* Contain a hyphen.
* Start with a letter (a-z).
@debugging
Rename your component's selector so that it matches the requirements.
**Before:**
```typescript
@Component({
selector: 'comp',
encapsulation: ViewEncapsulation.ShadowDom
...
})
```
**After:**
```typescript
@Component({
selector: 'app-comp',
encapsulation: ViewEncapsulation.ShadowDom
...
})
```