Problems about ISerializationCallbackReceiver In Unity Script API - unity3d

Question

I am learning something about Serialization in Unity, and know that ISerializationCallbackReceiver can be used to help serialize some complex data structure that cannot serialize directly. I have tested the following code in Unity 2017 and Find some problems.
public class SerializationCallbackScript : MonoBehaviour, ISerializationCallbackReceiver
{
public List<int> _keys = new List<int> { 3, 4, 5 };
public List<string> _values = new List<string> { "I", "Love", "Unity" };
//Unity doesn't know how to serialize a Dictionary
public Dictionary<int, string> _myDictionary = new Dictionary<int, string>();
public void OnBeforeSerialize()
{
_keys.Clear();
_values.Clear();
foreach (var kvp in _myDictionary)
{
_keys.Add(kvp.Key);
_values.Add(kvp.Value);
}
}
public void OnAfterDeserialize()
{
_myDictionary = new Dictionary<int, string>();
for (int i = 0; i != Math.Min(_keys.Count, _values.Count); i++)
_myDictionary.Add(_keys[i], _values[i]);
}
void OnGUI()
{
foreach (var kvp in _myDictionary)
GUILayout.Label("Key: " + kvp.Key + " value: " + kvp.Value);
}
}
Obviously, this example show me how to serialize a dictionary by changing it into a list and resuming when deserialize. However, when i run the codes pratically, I find _keys and _values both empty (keys.Count = 0) in inspector. Even worse, i cannot modify their values in the inspector. As a result, nothing appears when entering the play mode.
So I want to know what is the actual usage of ISerializationCallbackReceiver and the reason of what happened.

Answer 1

OnBeforeSerialize is called in the editor every time the inspector updates (basically always), and it calls clear on your keys and value lists
i changed this for me like that:
#if UNITY_EDITOR
using UnityEditor;
#endif
public class ....{
public void OnBeforeSerialize() {
#if UNITY_EDITOR
if(!EditorApplication.isPlaying
&& !EditorApplication.isUpdating
&& !EditorApplication.isCompiling) return;
#endif
...
so it does not clear on edit-time.