NullReference Exception when adding a class

Question

Hello everyone. I'm developing a shooting game with different types of bullets (pressing 1, 2, 3 and 4 will change them). I have a BasicBullet.cs which is like this

 using UnityEngine;
 using System.Collections;
 
 public class BasicBullet : MonoBehaviour
 {
     public float speed = 5.0f;
     public float DestroyTime = 10.0f;
     public Vector3 velocity;
     
     public void changeVelocity(Vector3 v)
     {
         velocity = v;
     }
 
     void Start ()
     {
         Destroy (gameObject, DestroyTime);
     }
 
 
     void FixedUpdate ()
     {
         // TO DO
         transform.position += velocity * Time.deltaTime;
     }
     
 }

And my second bullet is like this using UnityEngine; using System.Collections;

 public class Bullet2 : MonoBehaviour {
 
     public float speed = 5.0f;
     public float DestroyTime = 10.0f;
     public Vector3 velocity;
     
     public void changeVelocity(Vector3 v)
     {
         velocity = v;
     }
     
     void Start ()
     {
         Destroy (gameObject, DestroyTime);
     }
     
     
     void FixedUpdate ()
     {
         // TO DO
         transform.position += velocity * Time.deltaTime;
     }
 }

which basically are the same, just for tests purposes. Now, in my player shooting part, this is the code that I have

 using UnityEngine;
 using System.Collections;
 
 public class Shoot : MonoBehaviour
 {
     public AudioClip shootAudio;
     private AudioSource audioSource;
     public GameObject bulletPrefab;
     public GameObject fireParticle;
     public float fireRate = 0.1f;
     
     private float nextFire = 0.0f;
     private bool shooting = false;
     private int gunMode = 1;
     
     void Start()
     {
         audioSource = GetComponent<AudioSource>();
         
     }
 
     // Update is called once per frame
     void Update ()
     {
         //use mouse to fire
         if (Input.GetButtonDown ("Fire1"))
             shooting = true;
         if (Input.GetButtonUp ("Fire1"))
             shooting = false;
         
         //use keyboard to change gun type
         if (Input.GetKey("1"))
             gunMode = 1;
         else if(Input.GetKey("2"))
             gunMode = 2;
         else if(Input.GetKey("3"))
             gunMode = 3;
         else if(Input.GetKey("4"))
             gunMode = 4;
         
         if (shooting)
         {
             if(gunMode == 1  && Time.time > nextFire)
             {
                 fireParticle.SetActive(false);
                 audioSource.PlayOneShot(shootAudio);
                 GameObject bullet = (GameObject)Instantiate(bulletPrefab,this.transform.position + this.transform.forward + this.transform.up, Quaternion.identity);
                 
                 BasicBullet bulletComp = bullet.GetComponent<BasicBullet>();
                 bulletComp.changeVelocity(this.transform.forward * 10f);
                 nextFire = Time.time + fireRate;
             }
             else if(gunMode == 2)
             {
                 fireParticle.SetActive(true);
             }
             //TO DO
             else if(gunMode == 3 && Time.time > nextFire)
             {
                 fireParticle.SetActive(false);
                 audioSource.PlayOneShot(shootAudio);
                 GameObject bullet = (GameObject)Instantiate(bulletPrefab,this.transform.position + this.transform.forward + this.transform.up, Quaternion.identity);
                 
                 Bullet2 bulletComp = bullet.GetComponent<Bullet2>(); //<----PROBLEM!!!
                 bulletComp.changeVelocity(this.transform.forward * 10f);
                 nextFire = Time.time + fireRate;
             }
             else if(gunMode == 4)
             {
                 
             }
         }
         else
         {
             if(gunMode == 2)
             {
                 fireParticle.SetActive(false);
             }
         }
     }
     
     void OnTriggerStay(Collider other)
     {
         //for Flamethrower
         if(gunMode == 2)
         {
             if (other.attachedRigidbody)
             other.attachedRigidbody.AddForce(this.transform.forward * 50f);
         }
     }
 }

but I get a NullReferenceException in here

 Bullet2 bulletComp = bullet.GetComponent<Bullet2>();
                 bulletComp.changeVelocity(this.transform.forward * 10f);
                 nextFire = Time.time + fireRate;

What is the problem? Can anyone help me? Thanks! The error is this one:

 NullReferenceException: Object reference not set to an instance of an object
 Shoot.Update () (at Assets/Scripts/Player/Shoot.cs:65)

Answer 1

Answer by raimon.massanet · Nov 27, 2013 at 09:34 AM

You are instantiating the same prefab (`bulletPrefab`) in both cases. Is that intended behavior? If the prefab does not contain a Bullet2 script, GetComponent will return null, and you will get a NullReferenceException.

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Answer 2

It looks like you're trying to create a bullet object, but grabbing a component script. Bad form. First, generate a GameObject called BulletA, and attach Bullet1 to it. Next, generate a GameObject called BulletB, and attach Bullet2 to it. Drag them to your prefabs folder and delete it from your scene. Next, add a public GameObject bullet; to your Shoot script. This will allow you to drag-and-drop your BulletA or BulletB prefab as the designated item. In the case you want to spawn both bullets, create 2 public GameObject bullet_a, bullet_b; handles. Now, when you've decided this object should spawn, do something like this:

 GameObject bullet = (GameObject) GameObject.Instantiate(bullet_a, transform.position, transform.forward);

That's a temporary handle to a freshly spawned copy of bullet_a summoned at the parent scripts position, pointing forward in the parents direction.

From here, you can either throw the bullet by initial velocity, or raycast-teleport inside the Bullet1 script. Remember to destroy the bullet (preferably within it's script) when it is deemed obsolete (out of game bound, collided, traveled for X seconds or meters).

This guy will get you going with tutorials and visual aids: ETeeskiTutorials(Youtube)

NullReference Exception when adding a class

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