There are different ways to solve that, depending of the situation or what we want to achieve.

Solution A : (Static class)

@Component({
   selector: 'app-component',
   template: '<ng-content></ng-content>',

})
export class App implements OnInit {
  constructor(private cdRef:ChangeDetectorRef) {}
  
  someField: boolean = false;

  @HostBinding('class.someClass') someField: boolean = false;

  ngOnInit() {
    this.someField = true; // set class `someClass` on `<app-component>`
    //this.cdRef.detectChanges(); 
  }
}

Solution B : (dynamic class)

If you want to add a dynamic class to your host element, you may combine your HostBinding with a getter as

@HostBinding('class') get class() {
    return aComponentVariable // the name of the class. 
}

Example:

HTML: app.component.html

<h3>Pick the color of the text component</h3>
<input type="radio" name="headingColor" [value]="'Blue'" id="Blue" [(ngModel)]="headingColor"><label for="Blue">Blue</label>
<input type="radio" name="headingColor" [value]="'Red'" id="Red" [(ngModel)]="headingColor"><label for="Red">Red</label>
<input type="radio" name="headingColor" [value]="'Green'" id="Green" [(ngModel)]="headingColor"><label for="Green">Green</label>

TYPESCRIPT: app.component.ts

import { Component, HostBinding } from '@angular/core';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {
  headingColor: string = 'Black';

  @HostBinding('class') get HeadingClass() {
    return this.headingColor
  }
}

STYLESHEET: app.component.css

:host.Black{
  color: Black;
}

:host.Green {color: Green}

:host.Blue {color: Blue}

:host.Red {color: Red}

Solution C : (Use of ElementRef property)

import {Component, OnInit, ElementRef} from '@angular/core';

@Component({
  selector: 'my-app',
  template: '<ng-content></ng-content>'
})
export class AppComponent implements OnInit { 

 private currentIconClass: string;
get hostElement(): HTMLElement {
    return this._elementRef.nativeElement;
  }
  constructor(public _elementRef: ElementRef<HTMLElement>) {
  }
 ngOnInit() {
    this.setIconClass(this.hostElement.textContent);
  }
  private setIconClass(iconClass: string): void {

    if (!iconClass) {
      console.warn('Missing iconClass!');
      return;
    }

    this.clearElement();

    if (this.currentIconClass) {
      this.hostElement.classList.remove(this.currentIconClass);
    }

    this.currentIconClass = iconClass;

    this.hostElement.classList.add(this.currentIconClass);
  }
   private clearElement(): void {
    this.hostElement.textContent = undefined;
  }
}