<mat-form-field floatLabel="auto">
      <input [formControl]="displayNameControl"
        mongoIndexLimit
        [charLength]="charLength"
        matInput
        name="displayName"
        placeholder="Stack Name"
        autocomplete="off"
        required />
    </mat-form-field>

import { Directive, OnInit, NgModule, ElementRef, OnChanges, Input, SimpleChanges, Renderer2 } from '@angular/core';

@Directive({
  selector: '[mongoIndexLimit]'
})
export class MongoIndexLimitDirective implements OnInit, OnChanges {
  @Input() public charLength?: number;
  private maxLength = 5;
  constructor(
    private el: ElementRef<HTMLElement>,
    private renderer: Renderer2
  ) { }

  public ngOnInit() {
    this.el.nativeElement.setAttribute('maxLength', this.maxLength.toString());
  }

  public ngOnChanges(changes: SimpleChanges) {
    if (changes.charLength.currentValue >= 5) {
      const child = document.createElement('mat-error');
      this.renderer.appendChild(this.el.nativeElement.parentElement.parentElement.parentElement, child);
    }
  }

}