How can I access a script variable from another script in Unity?

Question

I'm creating a space shooter (think Space Invaders or Galaga) where the GameManager persists between scenes but the player and enemies don't. I need to access a boolean value from the player and enemies to define when and how the game should end. The PlayerController script contains a playerDead variable (attached to the Player GameObject) to tell the GameManager if the player has died and a enemiesDead variable to tell if all enemies have died.

Because the GameManager persists but the others don't, it loses the references when a new level is loaded. I tried to have the GameManager find the script references dynamically, to no avail. It always errors with Object reference not set to an instance of an object when a new level is loaded.

I've tried to get them with GameObject.FindGameObjectWithTag() and then using GetComponent<ScriptName>() to get the script and the variable in it. It compiles nicely and everything seems dandy, but then there's still no object reference. Am I just doing it wrong?

So how can I reference a script and get a value from it without even touching the Inspector?

EDIT:

Below is some code (everything else except the essential is scrapped) from the GameManager script, I tried to use Object.FindObjectsOfType() like this:

GameObject player;
GameObject enemies;

void Awake()
    {
        DontDestroyOnLoad (gameObject);
    }

void Start ()
    {
        Reinitialize ();
    }

void Update () 
    {
        if (enemies.GetComponent<EnemyGrid>().enemiesDead == true)
        {
            gameEnd = true;
        }

        if (player.GetComponent<PlayerController>().playerDead == true)
        {
            gameEnd = true;
        }
    }

void Reinitialize()
    {
        player = Object.FindObjectOfType(typeof(PlayerController)) as GameObject;
        enemies = Object.FindObjectOfType (typeof(EnemyGrid)) as GameObject;
    }

But there's still no object reference. My intention was to run the Reinitialize() funtion each time a new scene was loaded, so that way the GameManager would grab the new objects each time.

Answer 1

Getting null reference exception is normal since they were created in the previous scene and destroyed in the current one. To move them to the next scene, you need to call DontDestroyOnLoad.

Why don't you create your assets after the scene is loaded? GameManager should be responsible for creating player and enemy objects and keep track of the game state. In addition, try to use c# events to communicate with the manager in order to notify events such as EnemyDestroyed or PlayerDied etc. In this way, you don't need to ask both player and enemies whether they are dead at each frame.