Question about Unity serialize system

Question

Hi, i have a question about unity serialization system. these is pseudo code about problem.

 [System.Serializable]
 public class Parent
 {
     public int a;
     public Parent()
     {
         a = 111;
     }
 }
 
 [System.Serializable]
 public class Child : Parent
 {
     public int b;
     public Child()
     {
         b = 222;
     }
 }
 
 [System.Serializable]
 public class SOTest : ScriptableObject
 {
     public List<Parent> _database = new List<Parent>();
 }

SOTest class has List field and i add few instances that is child instance as below.

 public class test : MonoBehaviour
 {
     SOTest _database;
 
     void OnGUI()
     {
         if (GUILayout.Button("open"))
         {
             // load ScriptableObject
         }
         if (GUILayout.Button("create"))
         {
             // save ScriptableObject
         }
         if (GUILayout.Button("add"))
         {
             Parent test = new Child();
             _database._database.Add(test);
         }
         if (GUILayout.Button("show"))
         {
             for (int i = 0; i < _database._database.Count; i++)
             {
                 Child child = _database._database[i] as Child;
                 if (child != null)
                 {
                     Debug.Log(child.a + "  " + child.b);
                 }
                 else
                 {
                     Debug.Log("there is no child");
                 }
             }
         }
     }
 }

Problem is

i try to add child instances to database scriptableobject by push add button on screen.
then if i push show button, log shows me "111 222" that is exactly i expected.
then i save database object on disk.(serialization)
restart unity.
Load database object from disk.(deserialization)
push "show" button. BUT, Logs are "there is no child".

i think when i use generic list, deserialization from disk will retrieve exactly type of List. so if i add some kind of derived instance to list then serialization will lose derived class information. Am i right?

if i am wrong, how can fix this problem

thank you.

Answer 1

This is full code i tested.

 [System.Serializable]
 public class Parent
 {
     public int a;
     
     public Parent()
     {
         a = 111;
     }
 }
 
 [System.Serializable]
 public class Child : Parent
 {
     public int b;
 
     public Child()
     {
         b = 222;
     }
 }
 
 [System.Serializable]
 public class SOTest : ScriptableObject
 {
     public List<Parent> _database = new List<Parent>();
 
     public static ScriptableObject Create(string folder, string file)
     {
         string fullPath = folder + "/" + file;
 
         SOTest database = AssetDatabase.LoadAssetAtPath(fullPath, typeof(SOTest)) as SOTest;
         if (database == null)
         {
             if (!AssetDatabase.IsValidFolder(folder))
             {
                 AssetDatabase.CreateFolder(null, folder);
             }
 
             database = ScriptableObject.CreateInstance<SOTest>() as SOTest;
             AssetDatabase.CreateAsset(database, fullPath);
             AssetDatabase.SaveAssets();
         }
 
         return database;
     }
 }
 
 public class test : MonoBehaviour
 {
     string _assetFullPath;
     string _assetFolder;
     string _assetFile;
     SOTest _database;
 
     void OnGUI()
     {
         if(GUILayout.Button("open"))
         {
             Open();
         }
         if (GUILayout.Button("create"))
         {
             Save();
         }
         if (GUILayout.Button("add"))
         {
             Parent aa = new Child();
             _database._database.Add(aa);
         }
         if(GUILayout.Button("show"))
         {
             for(int i=0; i<_database._database.Count; i++)
             {
                 Child child = _database._database[i] as Child;
                 if(child != null)
                 {
                     Debug.Log(child.a + "  " + child.b);
                 }
                 else
                 {
                     Debug.Log("there is no child");
                 }  
             }
         }
     }
 
 
     public void Open()
     {
         _assetFullPath = EditorUtility.OpenFilePanel("Open Database", Constants._item_Db_FullPath, "asset");
         if (_assetFullPath != "")
         {
             LoadDatabase();
         }
     }
 
 
     public void Save()
     {
         _assetFullPath = EditorUtility.SaveFilePanel("Create Database", Constants._item_Db_FullPath, "New Database", "asset");
         if (_assetFullPath != "")
         {
             LoadDatabase();
         }
     }
 
     public void LoadDatabase()
     {
         if (_assetFullPath != null)
         {
             string absolutePath = Path.GetDirectoryName(_assetFullPath);
 
             int index = absolutePath.IndexOf("Assets");
 
             _assetFolder = absolutePath.Substring(index);
             _assetFile = Path.GetFileName(_assetFullPath);
 
             // DB Load            
             if (_assetFolder != "" && _assetFile != "")
             {
                 _database = SOTest.Create(_assetFolder, _assetFile) as SOTest;
             }
         }
     }
 }

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