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How can I use inheritance in Unity effectively?

I have basic understanding of inheritance but since every script I want as a component on a game object has to inherit from MonoBehaviour things get tricky.
Let's say I want to have a base class called Character and I want Player and Enemy to derive from Character. How am I suppose to add Player and Enemy script to Player and Enemy objects without getting an error?
I also tried to create empty game object and add CharacterStats to it then populate this class with different character objects with different stats. Guess what, you can't use new keyword to create objects if the script derives from Monobehaviour.
Is there some tutorials about this topic to make it more clear?

Probably the thing to understand is that MonoBehaviour itself inherits Component - A MonoBehaviour IS a Component.
When you look at it as if Components are a part of a thing, it makes at least some sense that you can't just new a Component - could I make a new Arm? If I did, where would it got? But I could make a new Body body and then dobody.AddComponent<Arm>, right? Because then it's clear what happens to the Arm - it gets attached to the body.
Similarly you can't just new a Component because they're supposed to be a part of a GameObject. What you can do is to make a new GameObject and .AddComponent<>() to that object. Again, now it's clear where that Component is going.
I generally agree with UnholySheep's comment that you should prefer composition over inheritance because this generally makes your code more modular. That said, I definitely do also use inheritance.
The question you should be asking yourself when you're outlining your class is, "Is this new class a KIND of _____ or does this new class HAVE a _____." And it's easy I guess to say a Player is a Character and an Enemy is a Character, but do you need to subclass? Or would a Player just be a Character that also has a PlayerController? Maybe you could have an interface like IPlayerController and have a LocalPlayerController for the user and an AIPlayerController for the enemies? Lots of ways to problem-solve with programming.
Or maybe all Characters have a PlayerController and just the .activeControl bit is false for non-player characters. Then you could do things like ghost/spectate NPCs, etc.
But to your question specifically:
Let's say I want to have a base class called Character and I want Player and Enemy to derive from Character. How am I suppose to add Player and Enemy script to Player and Enemy objects without getting an error?
You would start with a Character script that inherits MonoBehaviour and implements any of the logic that is common to both classes:
public class Character : MonoBehaviour
{
// Your common character logic
}
Then you would have subclasses inherit Character:
public class Player : Character
{
// Your Player-specific logic here
}
and
public class Enemy : Character
{
// Your Enemy-specific logic here
}
Then you either add the Enemy and Player scripts to a GameObject in the editor or you can do it programmatically by getting a reference to the target GameObject (by setting the reference in editor or by creating a new GameObject in a script) and then you call targetGameObject.AddComponent<YourComponentToAdd>();.

Using inheritance well comes from planning and iterating. Perhaps your Character class doesn't work well as a class for your enemies. I tend to have the player in their own class (unless its multiplayer) with enemies using their own base class. Below is an example of some inheritance I use in a 2D game I'm working on.
Base Class:
Enemy.cs
public abstract class Enemy : MonoBehaviour
{
public Vector2 flyInPos;
public Vector2 startPos;
public bool isFlyingIn;
public float flyInDuration = 5000f;
public float flyInTime = 0f;
public abstract void Destroy();
public abstract void DestroyWithoutScore();
public abstract void Attack();
public abstract void Attack2();
public abstract void Attack3();
public abstract void FlyOut();
public virtual void FlyIn()
{
isFlyingIn = true;
}
}
Every enemy needs to have these elements. You'll notice most of the methods in my base class are marked abstract so that each class that inherits can have a unique set of Attacks, and Flying patterns. One thing to notice is that the base class is inheriting from MonoBehaviour. Here is how I implemented inheritance in one of my enemies. Sorry its still a WIP.
Inherited Class:
ShootingDrone.cs
public class ShootingDrone : Enemy
{
public GameObject projectileSpawner;
public GameObject projectile;
public void Start()
{
startPos = transform.position;
Scheduler.Invoke(Attack, 5f, gameObject);
}
public void Update()
{
if(isFlyingIn)
{
transform.position = Vector3.Lerp(startPos, flyInPos, flyInTime / flyInDuration);
flyInTime += Time.deltaTime;
}
}
public override void Attack()
{
Shoot();
Scheduler.Invoke(Shoot, 0.25f, gameObject);
Scheduler.Invoke(Shoot, 0.5f, gameObject);
}
public override void Attack2()
{
Attack();
}
public override void Attack3()
{
Attack();
}
public void Shoot()
{
GameObject newProjectile = Instantiate(projectile, projectileSpawner.transform);
newProjectile.transform.parent = null;
}
public override void FlyOut()
{
throw new System.NotImplementedException();
}
public override void DestroyWithoutScore()
{
throw new System.NotImplementedException();
}
public override void Destroy()
{
Destroy(gameObject);
}
}
The base class of Enemy inherits from MonoBehaviour. So when Shooting Drone inherits from Enemy it also is inheriting from MonoBehaviour.
For your problem of trying to add a new MonoBehaviour. The reason for this is that a MonoBehavior HAS to be attached to something. Imagine a Rigidbody with no GameObject, it doesn't work. If you wanted to make a "new" CharacterStats the way you would do that is:
CharacterStats stats = gameObject.AddComponent<CharacterStats>();
If you don't want your CharacterStats as a component, then simply remove the MonoBehavior inheritance from the class and instantiate it as new CharacterStats.
There are a variety of tutorials that cover inheritance, but depending on how new you are to the subject I would start with the Unity official inheritance tutorial. I do think this tutorial is too brief but I also like CircutStreams tutorial since it also mentions implementing Interfaces which can be a better solution to inheritance in many cases.

When I'm trying to set a value for a base class from a child class (inheritance) it does not work (Unity C#)

I'm creating an enemy (from EnemyCreator1 class) with both EnemyMove1 and MarkusEnemy scripts (EnemyMove1 is a parent class to MarkusEnemy class). In EnemyCreator1 class I set value mainState of the script EnemyMove1 to "CHASE", but when I'm trying to access it from that class it says that mainState is "IDLE" (Please read my coments below because there are more explanations about what am I trying to achieve)
public class EnemyMove1 : MonoBehaviour
{
public enum mainStates { IDLE, CHASE }
public mainStates mainState;
void Update()
{
Debug.Log(mainState); //mainstate == IDLE, but should be CHASE
}
}
public class EnemyCreator1 : MonoBehaviour
{
[SerializeField] private GameObject enemyPrefab;
public void CreateEnemyAndSetItsStateToChase()
{
GameObject enemy = Instantiate(enemyPrefab);
enemy.GetComponent<EnemyMove1>().mainState = EnemyMove1.mainStates.CHASE;
}
}
public class MarkusEnemy : EnemyMove1
{
void Update()
{
EnemyMove enemyMoveScript = GetComponent<EnemyMove>();
Debug.Log(enemyMoveScript.mainState); //mainstate == CHASE
}
}

From the above code it looks like you are inheriting from a different base class EnemyMove, not EnemyMove1.

Thank you guys for helping me, after searching for the information about base classes I decided that it is impossible to access directly it's variables from another objects' scripts so I just simply call methods with variables as arguments (I put variables in round brackets of the method)

Unity calling singleton which is from another scene without using dontDestroyOnLoad

I have singleton( which is monobehavior also same question for non-monobehavior) which is created at scene 1 , and it created without dontDestroyOnLoad. im calling this singleton from scene 2 , and getting/using the info inside without any problem.I have read something about ghost GameObjects in this case but couldnt find detailed info.
In Scene 1
using UnityEngine;
public class RefreshAccount : MonoBehaviour
{
public static RefreshAccount refreshAccount;
public string aString = "aaaaaaaa";
void Awake()
{
if (!refreshAccount) refreshAccount = this;
else Destroy(this.gameObject);
// it is not labeled as DontDestroyOnLoad
}
(...)
}
Scene 2
using UnityEngine;
using UnityEngine.SceneManagement;
public class testnewscen : MonoBehaviour
{
private void Start()
{
Debug.Log(RefreshAccount.refreshAccount.aString );
}
}
So will it cause any problem/error in the future of this app ?
Will there be any memory problem or performance problem?

If you use this solution, you cannot run scene 2 without run scene 1 before
If you dont need to set serialize variable / game object / prefabs to your singleton (RefreshAccount) I prefer to use non-monobehaviour singleton instead like
public class RefreshAccount {
private static RefreshAccount instance
public static RefreshAccount Instance {
get {
if(instance == null) {
instance = createInstance();
}
return instance;
}
}
}
If you need to use for read some serialize value (variable, config, gameobject, etc.) without behaviour ( awake, update, fix update )
You can use SerializableObject
SerialzpizeObject is similar static class or prefab but you need to use RESOURCE.LOAD to read it

Why am I able to use variables inside a null reference?

I'm not really sure how to describe it exactly so let me show you what is going on.
I have a PlayerControls script which looks like this (note: I stripped everything except for the necessities).
namespace Player.Controls {
internal class PlayerControls: MonoBehaviour {
public bool IsClimbing { get; private set; } = false;
public bool IsGrounded { get; private set; } = false;
}
}
These variables are set in this class depending if the player is climbing/touching the ground. This script resides on the "Player" GameObject in the scene.
I have another script called PlayerControllerwhich looks like this
using Player.Controls;
public class PlayerController: Singleton<PlayerController> {
internal PlayerStats stats = new PlayerStats();
//PlayerStats nested class (see below)
}
The Singleton class only checks if the generic type is null, if it is, it will use FindObjectOfType to get an instance. This script also resides on the "Player" GameObject.
Inside the PlayerController class, I have a nested class called PlayerStats. It looks like this
internal class PlayerStats : PlayerControls {
public new bool IsClimbing { get { return base.IsClimbing; } }
public new bool IsGrounded { get { return base.IsGrounded; } }
}
Notice this nested class in inheriting from PlayerControls.
The idea is that the PlayerControls class in inaccessible to all other classes except for PlayerController, and any information I want to obtain regarding the player can be obtained by getting the player's instance (via the singleton) and accessing the PlayerStats variable.
For example, assuming the variable inside Singleton which holds the instance is called Instance, one could do PlayerController.Instance.stats.IsClimbing; Everything works as expected, except for one thing.
In the Awake method of the PlayerController class, I do this
private void Awake() {
Debug.LogFormat("In PlayerController Awake(). Is PlayerController.stats null? {0}",
(stats.Equals(null) ? "Yes" : "No"));
Debug.LogFormat("IsClimbing : {0}", stats.IsClimbing);
}
In the output window, it prints
In PlayerController Awake(). Is PlayerController.stats null? Yes
IsClimbing : False
If I also put the same IsClimbing debug in the Update() method, the value is correct for when I start climbing.
So, finally, my question, how can I access the variables of the PlayerStats class with the stats variable if stats is null? I thought it may have been somehow calling straight to the PlayerControls properties, so I changed their names, removed the new inside of PlayerStats and even put a debug statement inside one of the properties inside PlayerStats, and it definitely gets called. For example,public bool IsClimbing { get { Debug.Log("Called IsClimbing inside PlayerStats."); return base.Climbing; } }
If it is getting called and working properly, how can it be null? I asked my professor and he doesn't seem to know why either. What is really going on here?
Edit:
As requested, the Singleton class:
public abstract class Singleton<T>: MonoBehaviour where T : MonoBehaviour {
private static T instance;
public static T Instance {
get {
if(instance == null) {
instance = FindObjectOfType<T>();
}
return instance;
}
}
}
Here is an image of the console output.

Digging around on the Unity forums it appears that the Equals method has been overridden (on Object which MonoBehaviour eventually derives from) which is why comparing a MonoBehaviour to null is not giving you what you might expect. The answer I link to suggests code like this is more appropriate:
stats == null || stats.Equals(null)

how do I extend a class from another script in c# Unity

I'm trying to extend a base class on my player object.
player has damage script that looks like this
using UnityEngine;
using System.Collections;
public class Damage : MonoBehaviour
{
public int health = 100;
public virtual void ReceiveDamage(int damageAmount)
{
Debug.Log ("Original");
}
}
And then the same player has another script like this :
using UnityEngine;
using System.Collections;
public class playerDamage : Damage
{
public override void ReceiveDamage(int damageAmount)
{
Debug.Log ("Extended");
}
}
But when I call the script from a 3rd scrip on another object like this:
var damageScript = collision.gameObject.GetComponent<Damage>();
if( damageScript)
{
damageScript.ReceiveDamage(damageAmount);
}
the only response to the log is "Original"
Shouldn't the child be called and "Extended" written to the log?

There are several ways to do this. The easiest one is to SendMessage.
collision.gameObject.SendMessage("ReceiveDamage", damageAmount);
Whatever implementation of ReceivedDamage that the collision GameObject has, that will be the one that is called. This is awesome because you don't need to specify the type yourself nor use GetComponent.
Important Extra Information
In any implementation that you choose the key step is to make sure that the right script is attached to the collision.gameObject. If you attach both scripts then you are playing with fire.
To avoid playing with fire please make Damage an abstract class.
public abstract class Damage : MonoBehaviour
{
public int health = 100;
public virtual void ReceiveDamage(int damageAmount)
{
Debug.Log ("Original");
}
}
Abstract will give you the same functionality you want, except that Unity3d won't let you attach Damage to the GameObjects, which is good to avoid mistakes. You will always have the option to have the original ReceiveDamage and the choice to override it on future classes that inherit from Damage, like this:
public class Example : Damage
{
// This one still has ReceiveDamage but it happens in the base class.
}
or
public class PlayerDamage : Damage
{
public override void ReceiveDamage(int damageAmount)
{
Debug.Log("Extended");
}
}

I think when you call this line:
var damageScript = collision.gameObject.GetComponent<Damage>();
It gets the component by name, which in this case would be the Damage script, not the playerDamage script.
In you script it should be:
var damageScript = collision.gameObject.GetComponent<playerDamage>();
if( damageScript)
{
damageScript.ReceiveDamage(damageAmount);
}