Inventory.cs

using UnityEngine;
using System.Collections;

public class Inventory : MonoBehaviour {

    // Self explanatory..
    public int InventoryWidth = 400;
    public int InventoryHeight = 400;
    public Item StartingItem; // The item the player start with.

    private ItemStack LeftHand; // Item the player is holding in left hand, this variable isn't used for anything yet.
    private ItemStack RightHand; // Item the player is holding in right hand, this variable isn't used for anything yet.

    private ItemStack[] Items = new ItemStack[26]; // Array of itemstacks. This holds the player's items.

    private ItemStack SelectedItem; // When you click on an item it becomes the selected item, and info about it is
                    // shown in a box at the top.

    private bool InventoryOpen = false;
    private bool SeeingItem = false; // Just a bool for if the box with info about item in it is showing or not.

    void Start() {
        AddItem(StartingItem);
    }

    public void AddItem(Item item) {

                // Check if a stack is already there for us to stack the item with
            for(int i = 0; i <= 25; i++) {

                    // If nothing is in this inventory slot, check the next one.
                    if(Items[i] == null) {
                        continue;
                    }

                        // If stack is found, increase the stack size.
                    if(Items[i].ContainingItem.Name == item.Name) {
                        Items[i].StackSize++;
                        return;
                    }

            }

                // Create a new item stack.
            ItemStack st = new ItemStack(item, 1);

                // If no stacks of the same item is found, then
                // check for space in inventory to put a new item stack into.
            for(int i = 0; i <= 25; i++) {
                        // If item in inventory slot is null (no itemstack inside)
                    if(Items[i] == null) {
                                // Add a new item stack.
                        Items[i] = st;
                        return;
                    }
            }

            /*foreach(ItemStack i in Items) {
                    Debug.Log(i.ContainingItem.Name);
            }*/
        }

    void Update() {
        if(Input.GetKeyDown(KeyCode.E)) {
            InventoryOpen = !InventoryOpen;
            SeeingItem = false;
        }
    }

    void OnGUI() {
        if(InventoryOpen) {
            var pos = new Vector2(Screen.width / 2 - InventoryWidth / 2, Screen.height / 2 - InventoryHeight / 2);



            if(SeeingItem) {
                pos.y += 75;
                GUI.Box( new Rect(pos.x, pos.y - 110, InventoryWidth, 100), "Selected Item: " + SelectedItem.ContainingItem.Name );
            }

            // Just some fancy maths for the rest of the script, doesn't have anything to do with my problem
            GUI.Box( new Rect(pos.x, pos.y, InventoryWidth, InventoryHeight), "Inventory" );

            var buttonnum = 0;
            for(int y = 1; y <= 5; y++) {
                for(int x = 1; x <= 5; x++) {
                    buttonnum++;
                    var posx = pos.x + x * 60;
                    var posy = pos.y + y * 60;
                    if(x == 1) {
                        posx = pos.x + x * 55;
                    }
                    if(y == 1) {
                        posy = pos.y + y * 55;
                    }
                    if( Items[buttonnum] == null ) {
                        // NO ITEM
                        if( GUI.Button(new Rect(posx, posy, 50, 50), "" ) ) {
                            // DO NOTHING
                        }
                    }else{
                        if( GUI.Button(new Rect(posx, posy, 50, 50), Items[buttonnum].ContainingItem.Icon ) ) {
                            SelectedItem = Items[buttonnum];
                            SeeingItem = true;
                        }
                    }
                }
            }



            buttonnum = 0;
        }
    }
}