You are trying to create a MonoBehaviour using the 'new' keyword. This is not allowed.

You are trying to create a
MonoBehaviour using the ‘new’ keyword.
This is not allowed. MonoBehaviours
can only be added using
AddComponent(). Alternatively, your
script can inherit from
ScriptableObject or no base class at
all

I am trying to connect these two scripts and I don’t know which one this error is for. The full error is: You are trying to create a MonoBehaviour using the ‘new’ keyword. This is not allowed. MonoBehaviours can only be added using AddComponent(). Alternatively, your script can inherit from ScriptableObject or no base class at all
UnityEngine.MonoBehaviour:.ctor()
ItemClass:.ctor()
inventoryV1:.ctor()

Here are the two scripts

sing UnityEngine;
using System.Collections;

public class ItemClass : MonoBehaviour
{
//DECLARE ITEMS HERE
//MUST BE PUBLIC

//DECLARE ICONS HERE
//MUST BE STATIC AND PUBLIC
public class ItemCreatorClass
{
	string name;
	string description;
	int id;
	int damage;
	int speed;
	int weight;
	int hungerEffect;
	int thirstEffect;
	int stackAmount;
	Texture2D icon;
	
	public ItemCreatorClass(int ide,string nam, string des, int dam, int spe, int wei, int hun, int thi, int sta, Texture2D ico)
	{
		id = ide;
		name = nam;
		description = des;
		damage = dam;
		speed = spe;
		weight = wei;
		hungerEffect = hun;
		thirstEffect = thi;
		stackAmount = sta;
		icon = ico;
		
	}
}

}

using UnityEngine;
using System.Collections.Generic;

public class inventoryV1 : MonoBehaviour {
	public static bool inventoryOn = false;
	//GUI FONT AND SIZE
	private Vector2 windowPosition = new Vector2(300,0);
	private Vector2 windowSize = new Vector2(745, 750);
	
	private Vector2 closePosition = new Vector2(705,9);
	private Vector2 closeSize = new Vector2(24,24);
	
	private Vector2 inventoryTitlePosition = new Vector2(245,11);
	private Vector2 inventoryTitleSize = new Vector2(125,50);
	
	private Vector2 playerTitlePosition = new Vector2(17,11);
	private Vector2 playerTitleSize = new Vector2(100,50);
	
	private Vector2 craftingTitlePosition = new Vector2(17,414);
	private Vector2 craftingTitleSize = new Vector2(125,40);
	
	//Dictionary
	static public Dictionary<int, string> inventoryNameDictionary;
	
	//WINDOW TEXTURES
	public Texture inventoryWindow;
	
	//BUTTON TEXTURES
	public Texture closeInventory;
	public Texture inventorySlot;
	
	//ACCESS CLASSES
	ItemClass itemObject = new ItemClass();
	
	
	void Update ()
	{
		//ON OR OFF
		if (Input.GetKeyUp (KeyCode.Tab))
		{
			if (inventoryOn == false)
			{
				inventoryOn = true;
			}
			else if (inventoryOn == true)
			{
				inventoryOn = false;
			}
		}
	}
	//GUI AND FUNCTIONALITY
	void OnGUI()
	{
		if (inventoryOn == true)
		{
			GUILayout.BeginArea(new Rect (windowPosition.x, windowPosition.y, windowSize.x, windowSize.y), inventoryWindow);
			//Close Button
			if (GUI.Button (new Rect (closePosition.x, closePosition.y, closeSize.x, closeSize.y), closeInventory)) {
				inventoryOn = false;
			}
			//INVENTORY SECTION TITLES
			GUI.Label(new Rect(inventoryTitlePosition.x, inventoryTitlePosition.y, inventoryTitleSize.x, inventoryTitleSize.y),"Inventory");
			GUI.Label (new Rect( playerTitlePosition.x, playerTitlePosition.y, playerTitleSize.x, playerTitleSize.y), "Player");
			GUI.Label(new Rect(craftingTitlePosition.x, craftingTitlePosition.y, craftingTitleSize.x, craftingTitleSize.y), "Crafting Menu");
			//INVENTORY SLOTS
			//FIRST ROW
			GUI.Button (new Rect (270, 50, 70, 70), inventoryNameDictionary[0]);
			GUI.Button (new Rect (345, 50, 70, 70), inventoryNameDictionary[1]);
			GUI.Button (new Rect (420, 50, 70, 70), inventoryNameDictionary[2]);
			GUI.Button (new Rect (495, 50, 70, 70), inventoryNameDictionary[3]);
			GUI.Button (new Rect (570, 50, 70, 70), inventoryNameDictionary[4]);
			GUI.Button (new Rect (645, 50, 70, 70), inventoryNameDictionary[5]);
			//SECOND ROW
			GUI.Button (new Rect (270, 125, 70, 70), inventoryNameDictionary[6]);
			GUI.Button (new Rect (345, 125, 70, 70), inventoryNameDictionary[7]);
			GUI.Button (new Rect (420, 125, 70, 70), inventoryNameDictionary[8]);
			GUI.Button (new Rect (495, 125, 70, 70), inventoryNameDictionary[9]);
			GUI.Button (new Rect (570, 125, 70, 70), inventoryNameDictionary[10]);
			GUI.Button (new Rect (645, 125, 70, 70), inventoryNameDictionary[11]);
			//THIRD ROW
			GUI.Button (new Rect (270, 200, 70, 70), inventoryNameDictionary[12]);
			GUI.Button (new Rect (345, 200, 70, 70), inventoryNameDictionary[13]);
			GUI.Button (new Rect (420, 200, 70, 70), inventoryNameDictionary[14]);
			GUI.Button (new Rect (495, 200, 70, 70), inventoryNameDictionary[15]);
			GUI.Button (new Rect (570, 200, 70, 70), inventoryNameDictionary[16]);
			GUI.Button (new Rect (645, 200, 70, 70), inventoryNameDictionary[17]);
			//FOURTH ROW
			GUI.Button (new Rect (270, 275, 70, 70), inventoryNameDictionary[18]);
			GUI.Button (new Rect (345, 275, 70, 70), inventoryNameDictionary[19]);
			GUI.Button (new Rect (420, 275, 70, 70), inventoryNameDictionary[20]);
			GUI.Button (new Rect (495, 275, 70, 70), inventoryNameDictionary[21]);
			GUI.Button (new Rect (570, 275, 70, 70), inventoryNameDictionary[22]);
			GUI.Button (new Rect (645, 275, 70, 70), inventoryNameDictionary[23]);
			
			GUILayout.EndArea ();
			//ARMOR SLOTS
			
			
			//DICTIONARY
			inventoryNameDictionary = new Dictionary<int, string>()
			{
				//THE LINE BENEATH THIS COMMENT HAS THE ERROR
				{0, string.Empty},
				{1, string.Empty},
				{2, string.Empty},
				{3, string.Empty},
				{4, string.Empty},
				{5, string.Empty},
				{6, string.Empty},
				{7, string.Empty},
				{8, string.Empty},
				{9, string.Empty},
				{10, string.Empty},
				{11, string.Empty},
				{12, string.Empty},
				{13, string.Empty},
				{14, string.Empty},
				{15, string.Empty},
				{16, string.Empty},
				{17, string.Empty},
				{18, string.Empty},
				{19, string.Empty},
				{20, string.Empty},
				{21, string.Empty},
				{22, string.Empty},
				{23, string.Empty}
				
			};
			
		}
		
	}
}

The problem is, as has been said, this line:

ItemClass itemObject = new ItemClass();

Unity does not allow you to instantiate anything inheriting from the MonoBehaviour class using the new keyword. This seems a bit odd, until you consider what the MonoBehaviour class is.

MonoBehaviours are scripts that are attached to an object in the scene, and run in the scene as long as the object they are attached to is active. MonoBehaviours HAVE to be attached to something, or they won’t be able to function properly. When you use new, you are basically saying: “Please make one of these, store it somewhere, and give me a link to it”. What you don’t tell it is: “Also attach it to the place I’m calling it from”. The reason this isn’t done is because the concept of attaching things to objects is Unity specific, while the keyword new is general to C#, and so has no concept of Unity constructions - it can’t physically do this.

So how DO you specify what to attach it to? Well, Unity provides it’s own method for this - namely GameObject.AddComponent(). What this does is create a new script, of type T, and add it to the specified GameObject. So, if you have a GameObject called obj and a script called MyScript, you can dynamically add the script to your object by, instead of doing this:

MyScript script = new Script();

Which would, hypothetically, give you a script floating in space, not attached to anything, you can do this:

MyScript script = obj.AddComponent<MyScript>();

This will add a new MyScript component to obj, and then will set the variable script to that component, so you can then access it.

Hopefully, you can see why this issue occurs, and what you can do to solve it, now.

The issue is line 33:

ItemClass itemObject = new ItemClass();

The solution is to remove ‘Monobehaviour’ from the class declaration so this:

public class ItemClass : MonoBehaviour { //DECLARE ITEMS HERE //MUST BE PUBLIC

becomes this:

public class ItemClass { //DECLARE ITEMS HERE //MUST BE PUBLIC

Hello, if you want to call class from other file or other script it is easy. All script classes must inheritance from MonoBehaviour, and file and class name must be the same. You can make class members, or methods static and call “Classname”.“Method or member name”.

I think your question is formatted oddly, the problem however is the line

ItemClass itemObject = new ItemClass();

While this is the standard way of instantiating a class in C#, Unitys MonoBehaviours have their own way of instantiating, since scripts are components in Unity. gameObject.AddComponent(“ItemClass”);
Your ItemClass class is a MonoBehavior. ( public class ItemClass : MonoBehaviour )

Resurrecting this for those who believe this is an issue.

Just because its a unity warning does not mean your code is incorrect.

To list an example outside of your case, using async Tasks correctly will inevitably prompt a Unity Warning indicating a method was not “awaited”. For some scenarios this warning is true, but in cases of using parallelism Unity is wrong on this part.

As stated previously, writing “new” on a class that inherits the Monobehavior Class will always prompt a warning. The warning is not a problem. If your intention is to initialize and set reference to a script without creating a GAMEOBJECT, calling “new” is completely fine.
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The current voted Answer is suggesting to use AddComponent. Although this gets the job done the “UNITY WAY”, it is wasteful and creates a physical gameobject in your scene with your Added Component.