function getMyComponent(hostFixture: ComponentFixture<any>): MyComponent {
  const debugElement = hostFixture.debugElement.query(By.directive(MyComponent));

  return (debugElement && debugElement.componentInstance) || null;
}

const instance: MyComponent = getDirective<MyComponent>(fixture);

function getDirective<T>(hostFixture: ComponentFixture<any>): T {
  const debugElement = hostFixture.debugElement.query(By.directive(T));

  return (debugElement && debugElement.componentInstance) || null;
}

function getDirective<T>(
  hostFixture: ComponentFixture<any>,
  directive: new (...args: any[]) => T
): T {
  const debugElement =
  hostFixture.debugElement.query(By.directive(directive));

  return (debugElement && debugElement.componentInstance) || undefined;
}