[SOLVED] List.Exists / List.Any return NRE when List doesn't has object

Hello guys, I meet another problem.
I has 3 lists with items: factoryItems, productionItems, supplierItems
When player click on item script should check all 3 lists and find needed item.
My items are scriptable object, and I use itemID variable setted by hand

When player click on item he call Use() functions, here it is

Code (CSharp):

1. public override void Use()
2.     {
3.         base.Use();
4.  
5.         Debug.Log("ItemID: " + itemID);
6.  
7.        Item item = Inventory.instance.GetItemByID(itemID);
8.        if (item)
9.         {
10.             if(Inventory.instance.CheckItem(item))
11.             {
12.                 Debug.Log("Item is here");
13.             }
14.             Debug.Log("You have this item " + itemTitle);
15.             Debug.Log("Items need for craft " + prodList.Count);
16.             int resourceQty = prodList.Count;
17.             int resQty = 0;
18.             foreach(Item res in prodList)
19.             {
20.                 if(Inventory.instance.CheckItem(res))
21.                 {
22.                     resQty += 1;
23.                 }
24.             }
25.  
26.             if(resQty == resourceQty)
27.             {
28.                 Debug.Log("Succes, you can craft " + itemTitle);
29.                 int value = craftMultiplier;
30.                 Inventory.instance.AddItem(item, value);
31.                 foreach(Item res in prodList)
32.                 {
33.                     Inventory.instance.RemoveItem(res, 1);
34.                 }
35.             }
36.             else { Debug.Log("You don't have enough resource"); }
37.         }
38.        else
39.         {
40.             Debug.Log("You haven't " + itemTitle);
41.         }
42.     }

In this string
Item item = Inventory.instance.GetItemByID(itemID);
script call Inventory function, take itemID and give me object.

GetItemByID(itemID) firstly try to find item in player inventory, if yes just add item amount, if no add item to iventory

Code (CSharp):

1. public Item GetItemByID(int itemID)
2.     {
3.         Debug.Log("GetItemByID called, itemID: " + itemID);
4.         if (CheckItemByID(itemID))
5.         {
6.             return factoryItems.Find(x => x.itemID == itemID);
7.         }
8.         else
9.         {
10.             return CheckItemType(null, itemID);
11.         }
12.     }

As you can see firstly I check item if it exists. I this part it the problem, I got NRE here:

Code (CSharp):

1. public bool CheckItemByID(int itemID)
2.     {
3.         Debug.Log("Try to check item by ID " + itemID);
4.         return factoryItems.Any(x => x.itemID == itemID);
5.     }

I tried to use .Exists and .Any, but both methods crash with NRE.

How I should check List for object if I know only object variable and don't have object itself?

 

If you get NRE in this particular code section, it means that either the

factoryItems

list is null or any element in it, as you try to access a property of an object that does not exist.

Try this, although this is probably not fixing the underlying problem of something being null.

return factoryItems.Any(x => x != null && x.itemID == itemID);

As a little tip, you can improve this code. Right now, you go through the list of factoryItems to check if an item has the ID, then if it is true, then you go through the list again to return it. Try to do it like this:

Code (CSharp):

1.     public bool CheckID(int itemID){
2.     return factoryItems.Find(x => x != null && x.itemID == itemID);
3.     }
4.  

Code (CSharp):

1.     public Item GetItemByID(int itemID)
2.         {
3.             Debug.Log("GetItemByID called, itemID: " + itemID);
4.             Item item = CheckItemByID(itemID);
5.             if (item != null)
6.             {
7.                 return item;
8.             }
9.             else
10.             {
11.                 return CheckItemType(item, itemID);
12.             }
13.         }
14.  

There are also other solutions using the out parameter modifier, with a popular example being Int32.TryParse.

 

Greate thanks man! It works, and I need null if element doesn't exist. Because factoryItems work like player inventory and if player hasn't item, I get it from another list, get as object and add to factoryItems.

I understand that you advice me, I'll improve my code for lesser queries to lists, actually for now I can don't do List.Exists and always use List.Find.