[Unity] SerializedInterface 2020

SerializableInterfaceDrawer.cs

[CustomPropertyDrawer(typeof(Object<>))]
public class SerializableInterfaceDrawer : PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        var serializedValueProperty = property.FindPropertyRelative("serializedValue");

        var interfaceType = fieldInfo.FieldType.GetGenericArguments()[0];

        serializedValueProperty.objectReferenceValue =
            EditorGUI.ObjectField(position, label, serializedValueProperty.objectReferenceValue, interfaceType, true);
    }
}

SerializedInterface.cs

using UnityEngine;
using Object = UnityEngine.Object;

[System.Serializable]
public class Object<TInterface> where TInterface : class
{
    [SerializeField]
    private Object serializedValue;

    private TInterface value = null;
    public TInterface Value
    {
        get
        {
            if (value == null && serializedValue != null)
            {
                value = (TInterface)(object)serializedValue; // No memory allocation
            }
            return value;
        }
        set
        {
            serializedValue = (Object)(object)value;
            this.value = null; // To trigger new caching for next get call
        }
    }

    public static implicit operator TInterface(Object<TInterface> obj)
    {
        return obj.Value;
    }
}