Set Dirty on class instance?

Hello, people! Here is the problem:

I have 3 classes and 1 editor for classA.

 [Serializable]
 ClassA : Monobehavior{
      [SerializeField]
     ClassB varb = new ClassB( );
 }
  
 [Serializable]
 ClassB {
 [SerializeField]
 List<ClassC> listC = new List<ClassC>();
 }
  
 [Serializable]
 ClassC{
 [SerializeField]
 List<Vector3> pos;
 }

The thing is, when I change a value in Editor, I would attempt to go EditorUtility.SetDirty( (ClassA)target)); so that the changes are saved and won't be lost when I hit "Play"

But, how to I mark the variables in Class A, such as "varb", which represent instances of ClassB?

Saying EditorUtility.SetDirty( ((ClassA)target).varb ); will say :

Assets/Editor/NodeCounterEditor.cs(55,39): error CS1502: The best overloaded method match for UnityEditor.EditorUtility.SetDirty(UnityEngine.Object)' has some invalid arguments Casting ((ClassA)target).varb to Object will say Assets/Editor/NodeCounterEditor.cs(55,56): error CS0030: Cannot convert 'ClassB' expression to UnityEngine.Object'

Please do not mention ScriptableObject, I cannot use it in my context

Thank you very much for your help!