A unique ID for your Serialized classes in Unity.

MyClassDrawer.cs

[Serializable]
public class MyClass {
  public int id;
}

[CustomPropertyDrawer(typeof(MyClass))]
public class MyClassDrawer : PropertyDrawer
{
  public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
  {
    var idProperty = property.FindPropertyRelative("id");
    idProperty.intValue = property.propertyPath.GetHashCode();
  }
}

Note, the GetHashCode() method will probably return a unique ID. There is a very remote chance you will get a clashing ID.