Serialization: Objects referenced from SerializeReference fields now have stable ids, which reduces risk of conflicts when multiple users collaborate on a scene file. This also improves support for undo and prefabs, especially when SerializeReference is used inside arrays and lists. There is a new format for references, with backward compatibility support for older assets.

Serialization: SerializeReference now allow more granular handling of missing types. SerializeReference instances for which the type is missing will be replaced by null. Other instances will be editable and if fields who were previously referring to the missing type are still null the missing type will be preserved.

public class TestShapesForSRD : MonoBehaviour
{
    [SerializeReferenceDropdown]
    [SerializeReference]
    private IShape _singleShape;
    
    [SerializeReferenceDropdown]
    [SerializeReference]
    private IShape[] _shapesArray;
}

public interface IShape
{
}

[Serializable]
public class Circle : IShape
{
    [SerializeField]
    private float _radius;
}

[Serializable]
public class Rectangle : IShape
{
    [SerializeField]
    private float _sideA;
        
    [SerializeField]
    private float _sideB;
}

public class TestShapesForSRD : MonoBehaviour
{
    [SerializeReference, SerializeReferenceDropdown]
    private ISimpleGenericData<int> _intData;
}

public interface ISimpleGenericData<TData> : IAbstractData
{
    public TData Data { get; }
}

[Serializable]
public class GenericData<TData> : ISimpleGenericData<TData>
{
    [SerializeField] private TData _data;

    public TData Data => _data;
}

[Serializable]
public class GenericKeyValuePair<TKeyData, TValueData> : ISimpleGenericData<TKeyData>, IAbstractData
{
    [SerializeField] private TKeyData _key;
    [SerializeField] private TValueData _value;
    public TKeyData Data => _key;
}