namespace RogueBase.Classes
{
    public class Player
    {
        private int _hp;
        public int hp
        {
            get { return _hp; }
            set
            {
                if (value <= 0) _hp = 0;
                else if (value > 100) _hp = 100;
                else _hp = value;
            }
        }

        public Player()
        {
            hp = 100;
        }
    }
}

using RogueBase.Interfaces;
namespace RogueBase.Classes
{
    public class Storage
    {
        public Iitem[] inventory { get; set; }

        public Storage(int capacity)
        {
            inventory = new Iitem[capacity];
        }

        public bool Add(Iitem newItem)
        {
            for (int i = 0; i < inventory.Length; i++) // check each slot
            {
                if (inventory[i] as Iitem != null) { continue; } // skip items

                inventory[i] = newItem; // assign item to empty slot
                return true; // report success
            }
            return false; // inventory full
        }

        public bool Remove(Iitem targetItem)
        {
            for (int i = 0; i < inventory.Length; i++) // check each slot
            {
                if(inventory[i] == targetItem) // when match is found
                {
                    inventory[i] = null; // remove it
                    return true; // report success
                }
            }
            return false; // no item found
        }
    }
}

namespace RogueBase.Classes
{
    public class Player
    {
        public Iitem[] inventory { get; set; }

        private int _hp;
        public int hp
        {
            get { return _hp; }
            set
            {
                if (value <= 0) _hp = 0;
                else if (value > 100) _hp = 100;
                else _hp = value;
            }
        }

        public Player()
        {
            hp = 100;
        }
    }
}

using RogueBase.Interfaces;
namespace RogueBase.Classes
{
    public class Storage
    {
        public Storage(int capacity)
        {
            inventory = new Iitem[capacity];
        }

        public bool Add(Iitem newItem)
        {
            for (int i = 0; i < inventory.Length; i++) // check each slot
            {
                if (inventory[i] as Iitem != null) { continue; } // skip items

                inventory[i] = newItem; // assign item to empty slot
                return true; // report success
            }
            return false; // inventory full
        }

        public bool Remove(Iitem targetItem)
        {
            for (int i = 0; i < inventory.Length; i++) // check each slot
            {
                if(inventory[i] == targetItem) // when match is found
                {
                    inventory[i] = null; // remove it
                    return true; // report success
                }
            }
            return false; // no item found
        }
    }
}

Edit; Specified question:

How can I access Iitem[] inventory in class Player to be used in class Storage's methods?

I'm a beginner in C# and trying to implement a player, an item interface, and some kind of storage system (for player inventory, NPC inventory/drop list, and storage like chests), and having some trouble implementing it in a way that requires the least work for a large amount of class types (and instances).

The problem right now is that every time I want to create a new inventory for something, I need to prepare a variable for it (not a problem when it comes to the player), but I'd like to be able to create friendly and hostile NPCs with their own unique inventory that could be randomly generated on demand.

In order to do this, is it my only option to turn the Storage class into an interface and copypaste the functions for every NPC/storage type? Or can I somehow turn the Add/Remove functions static? If I try to it right now I get an error:

I'm a beginner in C# and trying to implement a player, an item interface, and some kind of storage system (for player inventory, NPC inventory/drop list, and storage like chests), and having some trouble implementing it in a way that requires the least work for a large amount of class types (and instances).

The problem right now is that every time I want to create a new inventory for something, I need to prepare a variable for it (not a problem when it comes to the player), but I'd like to be able to create friendly and hostile NPCs on demand with their own unique inventory that could be randomly generated,

In order to do this, is it my only option to turn the Storage class into an interface and copypaste the functions for every NPC/storage type? Or can I somehow turn the Add/Remove functions static? If I try to do it right now I get an error: