Custom Editor on Serialized Objects which are not derived from Monobehaviour

Hi... I cannot manage to have the desired behaviour on my custom editor when using objects which are not extending anything... Below is an oversimplified version of what I would like to achieve.

What I have :

• a serialized class MyClass holding an enum and a string

• a script object MyClassHolder, extending MonoBehaviour, holding various instances of MyClass

• a custom Editor MyClassEditor which should create a dynamic behaviour on "MyClass" properties

Behaviour wanted :

• I inspect MyClassHolder using the inspector

• I see the list of instances of MyClass

• I select one instance

• I select one of the value from the enum dropdown list

• I would like the string value to update using some calculation.

However the string value does not update... Below is the actual code I try to fix... Can anyone help me ??

 // file Assets/Scripts/MyClass.cs
 using UnityEngine;
 public class MyClassHolder : MonoBehaviour
 {
     public MyClass example1;
     public MyClass example2;
 }
 
 [System.Serializable]
 public class MyClass
 {
     public Dropdown dropdown;
     public string textFromDropdown;
     public enum Dropdown {ResponseOne, ResponseTwo }
 }

.

 // file Assets/Scripts/Editor/MyClassEditor.cs
 using UnityEditor;
 
 [CustomEditor(typeof(MyClass)), CanEditMultipleObjects]
 public class MyClassEditor : Editor
 {
     public SerializedProperty dropdown_prop;
     public SerializedProperty textFromDropdown_prop;
 
     void OnEnable()
     {
         dropdown_prop = serializedObject.FindProperty("dropdown");
         textFromDropdown_prop = serializedObject.FindProperty("textFromDropdown");
     }
 
     public override void OnInspectorGUI()
     {
         serializedObject.Update();
         EditorGUILayout.PropertyField(dropdown_prop);
         MyClass.Dropdown dropdownIndex = (MyClass.Dropdown)dropdown_prop.enumValueIndex;
 
         switch (dropdownIndex)
         {
             case MyClass.Dropdown.ResponseOne:
                 textFromDropdown_prop.stringValue = "I am Response One";
                 break;
             case MyClass.Dropdown.ResponseTwo:
                 textFromDropdown_prop.stringValue = "I am Response Two";
                 break;
         }
         serializedObject.ApplyModifiedProperties();
     }
 }