Custom Editor Inspector member class with inheritance, values resets after starting Play Mode

Hello, let's say I have following classes:

[System.Serializable]
public class A {
    public float a;
}

[System.Serializable]
public class B : A {
    public float b;
}

[System.Serializable]
public class C : A {
    public float c;
}

using UnityEngine;
public class Manager : MonoBehaviour {
    public enum Mode { B, C };
    public Mode mode = Mode.B;
    [SerializeField]
    public A classA;
}

 using UnityEngine;
 using UnityEditor;

 [CustomEditor(typeof(Manager))]
 public class ManagerEditor : Editor {
 
     private string[] tabNames = { "B", "C" };
     Manager myTarget;
 
     private void setClass() {
         if(myTarget.mode == Manager.Mode.B) {
             if (myTarget.classA.GetType() != typeof(B)) {
                 myTarget.classA = new B();
             }
             displayB();
         } else if (myTarget.mode == Manager.Mode.C) {
             if(myTarget.classA.GetType() != typeof(C)) {
                 myTarget.classA = new C();
             }
             displayC();
         }
     }
 
     private void displayC() {
         myTarget.classA.a = EditorGUILayout.FloatField("a", myTarget.classA.a);
         (myTarget.classA as C).c = EditorGUILayout.FloatField("c", (myTarget.classA as C).c);
     }
 
     private void displayB() {
         myTarget.classA.a = EditorGUILayout.FloatField("a", myTarget.classA.a);
         (myTarget.classA as B).b = EditorGUILayout.FloatField("b", (myTarget.classA as B).b);
     }
 
     public override void OnInspectorGUI() {
         myTarget = target as Manager;
         myTarget.mode = (Manager.Mode)GUILayout.Toolbar((int)myTarget.mode, tabNames);
         setClass();
     }
 }

Everything seams to be fine, but when I set some values in inspector in edit mode, for example mode C, and a = 1, c = 2:

as soon as I press Play, every value resets:

Is there any way to prevent that and keep values from edit mode in play mode or I am doing something wrong?