how to add C# script that creates and Instantiates a Game Object in Unity game

Question

I know that scripts are added as components to game objects but I have created a game object using C# script. This is the only game object in my simple test game. How should I "add" it to my game? Please see code below:

public class TestingHeroPositions : MonoBehaviour {

GameObject hero;
Sprite heroSprite; 

void Start () {

    heroSprite = Resources.Load <Sprite> ("Sprites/heroImage");
    SpriteRenderer renderer = hero.AddComponent<SpriteRenderer>();
    renderer.sprite = heroSprite;

    Camera camera = GetComponent<Camera>();
    Vector3 heroPosition = camera.ScreenToWorldPoint(new Vector3(Screen.width/2, Screen.height/2, camera.nearClipPlane));

    Instantiate (hero, heroPosition, Quaternion.identity);

    }

}

I am getting a NullReferenceException: Object reference not set to an instance of an object error pointing to the line SpriteRenderer renderer = hero.AddComponent<SpriteRenderer>(); — Jickery
– Jickery, Commented Apr 2, 2016 at 12:41
@Jickery Your hero gameobject is null... Either make it public and set it in inspector or use GameObject.Find — Martin Dawson
– Martin Dawson, Commented Apr 2, 2016 at 12:47
@MartinMazzaDawson is solved it by adding hero = new GameObject();. Though it's working but there's another error which says MissingComponentException: There is no 'Camera' attached to the "GameObject" game object, but a script is trying to access it. You probably need to add a Camera to the game object "GameObject". Or your script needs to check if the component is attached before using it. TestingHeroPositions.Start () pointing to the line Vector3 heroPosition = camera.ScreenToWorldPoint(new Vector3(Screen.width/2, Screen.height/2, camera.nearClipPlane)); — Jickery
– Jickery, Commented Apr 2, 2016 at 14:35
@Jickery The error is self explanatory. You're trying to access the camera component on this gameobject when their isn't one.. This line GetComponent<Camera>() — Martin Dawson
– Martin Dawson, Commented Apr 2, 2016 at 14:37

Aizen · Accepted Answer · 2016-04-02 17:14:53Z

2

You have 3 Major Problems. It is the flow of your code.

1 . You're adding a component to an Object Class not a GameObject Class. The AddComponent is a member class of GameObject.

Like the top problem, Since your hero is a GameObject. You are able to set AddComponent member of it. But the truth is, it is not Getting Instantiated yet.
Instiate is member of Object Class not GameObject class, thus it returns an Object class.

To solve.

public class TestingHeroPositions : MonoBehaviour {

GameObject hero;
Sprite heroSprite; 

void Start () {





    Instantiate (hero, heroPosition, Quaternion.identity) as GameObject;
     //Instantiate first then type cast it to GameObject. Instiate returns Object not gameObject.
     //No need for `new GameObject()` Constructor.

    Camera camera = GetComponent<Camera>();
    Vector3 heroPosition = camera.ScreenToWorldPoint(new Vector3(Screen.width/2, Screen.height/2, camera.nearClipPlane));
heroSprite = Resources.Load <Sprite> ("Sprites/heroImage");
        SpriteRenderer renderer = hero.AddComponent<SpriteRenderer>();
        renderer.sprite = heroSprite;

        }

    }

answered Apr 2, 2016 at 17:14

Aizen

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1 Comment

Jickery Over a year ago

Thanks, you have used heroPosition without declaring it, so it's giving a NullReferenceException error

Marko vlaić · Accepted Answer · 2016-04-08 10:00:19Z

1

Before instantiating the object add heroInstance.AddComponent<MonoBehaviour>(this);. This should work :)

edited Apr 8, 2016 at 10:00

answered Apr 2, 2016 at 12:20

Marko vlaić

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2 Comments

Aizen Over a year ago

No it wont. Because the object is not Instantiated and it is not casted to GameObject.

Marko vlaić Over a year ago

Sorry, than add ´heroInstance.AddComponent<MonoBehaviour>(this);´

Tobias Wehrum · Accepted Answer · 2016-04-02 12:29:27Z

Simple: You create a new game object in your scene, call it (for example) "Hero Spawner" and attach your "TestingHeroPositions" to the "Hero Spawner".

If you want to create multiple heroes, that is the way to go, although your script should be slightly different:

public class TestingHeroPositions : MonoBehaviour {

    GameObject heroPrefab;
    Sprite heroSprite; 

    void Start () {
        Camera camera = GetComponent<Camera>();
        Vector3 heroPosition = camera.ScreenToWorldPoint(new Vector3(Screen.width/2, Screen.height/2, camera.nearClipPlane));

        // Instantiate a new instance of heroPrefab into the scene
        var heroInstance = Instantiate (heroPrefab, heroPosition, Quaternion.identity);

        // Only add the hero sprite renderer to THIS instance of the hero Prefab
        heroSprite = Resources.Load <Sprite> ("Sprites/heroImage");
        SpriteRenderer renderer = heroInstance.AddComponent<SpriteRenderer>();
        renderer.sprite = heroSprite;
    }
}

If you only want to create one hero, you might want to consider letting the Hero object itself (via a component) decide which sprite it has. And if that sprite never changes, maybe just add it to your hero prefab.

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