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我在application Component的HTML里定义了一个router-outlet元素：

遇到错误消息：

router-outlet is not a known element

查看其定义，源代码比文档更有说服力：

/** * @description * * Acts as a placeholder that Angular dynamically fills based on the current router state. * * Each outlet can have a unique name, determined by the optional `name` attribute. * The name cannot be set or changed dynamically. If not set, default value is "primary". * * ``` * 
    * 
    * 
    * ``` * * A router outlet emits an activate event when a new component is instantiated, * and a deactivate event when a component is destroyed. * * ``` * 
    * ``` * @ngModule RouterModule * * @publicApi */export declare class RouterOutlet implements OnDestroy, OnInit {
       private parentContexts;    private location;    private resolver;    private changeDetector;    private activated;    private _activatedRoute;    private name;    activateEvents: EventEmitter
   
    ;    deactivateEvents: EventEmitter
    
     ;    constructor(parentContexts: ChildrenOutletContexts, location: ViewContainerRef, resolver: ComponentFactoryResolver, name: string, changeDetector: ChangeDetectorRef);    ngOnDestroy(): void;    ngOnInit(): void;    get isActivated(): boolean;    get component(): Object;    get activatedRoute(): ActivatedRoute;    get activatedRouteData(): Data;    /**     * Called when the `RouteReuseStrategy` instructs to detach the subtree     */    detach(): ComponentRef
     
      ;    /**     * Called when the `RouteReuseStrategy` instructs to re-attach a previously detached subtree     */    attach(ref: ComponentRef
      
       , activatedRoute: ActivatedRoute): void;    deactivate(): void;    activateWith(activatedRoute: ActivatedRoute, resolver: ComponentFactoryResolver | null): void;    static ɵfac: ɵngcc0.ɵɵFactoryDef
       
        ;    static ɵdir: ɵngcc0.ɵɵDirectiveDefWithMeta
        
         ;}
        
       
      
     
    
   

是一个place holder，Angular根据当前的router state在运行时填充。

解决办法：

在AppModule里导入RouteModule即可：

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