Why C# downcast become null?

Unity Engine
1

So here is my code,

public void CheckStatChal()    
{
   foreach (SpotUIBase menu in thisSpot.ownMenus)
   {
       if (menu.uiSort == SpotUISort.StatEvent)
       {
           if(menu != null)
               Debug.Log("Menu name is "+menu.Name);
           var statEvent = menu as StatEvent;
           if (statEvent == null)
           {
               Debug.Log("Stat event is null, name is "+thisSpot.Name);
               continue;
           }
           .......... [1]

public SpecialSpotClass thisSpot;
public abstract class SpecialSpotClass
{
public List<SpotUIBase> ownMenus = new List<SpotUIBase>();
....
public  class SpotUIBase
{
  public SpotUISort uiSort;
   ....
public class StatEvent : SpotUIBase
{
   ....
public enum SpotUISort{
   Inn, Shop, Bar,

I am using Unity engine now. So if run this code, I got Debug.Log("Menu name is "+menu.Name); and Debug.Log("Stat event is null, name is "+thisSpot.Name); both. Why? menu is not null, but after downcast it, it become null? I don’t understand this why.

So in this code, I want to execute [1] part below codes, but [statEvent] is null, so all the code below does not called by (continue keyword)

Why downcast become null?

Help please.

2

You’re using a safe cast (as), which returns null if it fails. This means that your menu object is not a StatEvent.

If you think the cast should always work, use a normal cast. Then you’ll get an exception from the cast, which is where you did something wrong, rather than a nullref on some arbitrary line further down.

3

It’s null because the cast fails. You’re trying to cast a base class into a derived class.

While a StatEvent is always a SpotUIBase, a SpotUIBase isn’t always a StatEvent.

4

The, how should it be? Eventually I want to access StatEvent’s own function.

5

Make the method public and any inherited class will be able to use it. That’s how inheritance works…

6

StatEvent’s method is already public, StatEvent’s null is problem.

7

If menu truly is a StatEvent then you can just call menu.Method(). There’s no need to cast. But given the fact that your safe cast is coming back null my guess is that menu isn’t a StatEvent in which case assuming that every menu is a StatEvent doesn’t seem to be a good assumption given your architecture.

Edit - now I sort of see what you’re doing. It’s not a downcast, it’s an upcast. So yeah, not every menu (SpotUIBase) is a StatEvent.

8

Just curious, but what reason are you making a list of SpotUIBase instead of a list of StatEvent? Are you trying to create a list that holds a bunch of different things that inherit from SpotUIBase?

9

So I googled and confirmed right Downcast method, and changed foreach to for syntax. And solved.

Here is changed code.

 public StatChal chal1;
for (int i = 0; i < thisSpot.ownMenus.Count; i++)
   {
       if (thisSpot.ownMenus[i].uiSort == SpotUISort.StatEvent)
       {
           thisSpot.ownMenus[i] = SpotUI.Instance.StatEvent;
           var ownMenu = (StatEvent) thisSpot.ownMenus[i];
           Debug.Log("own menu is "+ownMenu.Name);
           if ((!ownMenu.StatChal1.nowCoolTime && ownMenu.StatChal1 != null)
                                            || ownMenu.StatChal1 == null)
           {
               StatChallenge.Instance.MakeStatChal(this, ref ownMenu.StatChal1);
               Debug.Log(ownMenu.StatChal1.Name);
               ownMenu.SetChalInfo(ownMenu.StatChal1, 1);
               chal1 = ownMenu.StatChal1;
           }
10

So its started from game design, my game has various special in-game spots like Village, Dungeon, Ruins, etc.

And all of them will has its own specific menus. (ex. Village will has Inn, Shop, StatEvent UI menus)

So I made SpotUIBase class first and from there all specific ui menus (Inn, Shop,) inherited from it.

And at this thread, I want to make checking method that if some special spot has [StatEvent] UI menu, I want to pull specific [StatEvent] data from [StatEvent] manager class. (in my case, StatChallenge.cs)

11

Tested, but this does not worked well.

thisSpot.ownMenus = SpotUI.Instance.StatEvent;

So this line is not truly downcast itself, but just change its data.

So I just abandoned this downcasting approach and just get reference of each specific spot’s own script and store data to there.