SerializeField/Serializable doesn't work for list of custom class (List<customClass>)

Problem: my list of custom class (public List<ComponentClass> OnCast_Comp = new List<ComponentClass>();) doesn’t appear in the unity editor. I already have [SerializeField] above it, with the custom class having [System.Serializable] also above it. But it still wouldn’t show up in the editor.

SpellHolder.cs:

public class SpellHolder : MonoBehaviour
{
    // holds the currently owned spells for the player
    // only holds the spell, the content of the spell itself is inside SingleSpellManager.cs
    // [SpellHolder.cs] -> SingleSpellManager.cs

    [SerializeField]
    public List<SingleSpellManager> List_Spells = new List<SingleSpellManager>();
}

SingleSpellManager.cs:

[System.Serializable]
public class SingleSpellManager 
{
    // stores the magical components (MComp) of the spell 
    // SpellHolder.cs -> [SingleSpellManager.cs]

    [SerializeField]
    public List<ComponentClass> OnCast_Comp = new List<ComponentClass>();
    [SerializeField]
    public List<ComponentClass> OnHit_Comp = new List<ComponentClass>();

    [Header ("General Data")]
    public string spellName;
}

ComponentClass.cs:

// interface class
[System.Serializable]
public abstract class ComponentClass 
{
    public virtual void OnCast(){}
    public virtual void OnHit(){}

    //general info
    public string name = null;
    public string desc = null;

    //floats
    public float fl_a, fl_b, fl_c;

    //ints (can be used as types)
    public int int_a, int_b, int_c;

    //bools
    public bool bl_a, bl_b, bl_c;
}



// derivatives
[System.Serializable]
public class Projectile : ComponentClass
{
    // desc:
    // turns the spell into a projectile.

    public float power {get {return fl_a;}}
    
    public override void OnCast()
    {   // test
        UnityEngine.Debug.Log(power);
    }
}

>Solution :

You need use SerializeReference when you want to save different type of objects in one collection.

[System.Serializable]
public class SingleSpellManager 
{
    [SerializeReference]
    public List<ComponentClass> OnCast_Comp = new List<ComponentClass>();
    [SerializeReference]
    public List<ComponentClass> OnHit_Comp = new List<ComponentClass>();
}