- Home /
C# NullReferenceException
Hi,
I keep getting this error when I start my game:
NullReferenceException: Object reference not set to an instance of an object PlayerAttack.Start () (at Assets/Standard Assets/Scripts/PlayerAttack.cs:19)
And I don't know what to do with it. Basically I'm trying to copy the content from a List KnownNames in script PlayerKnowledge, into a new List<> Knowledge in script PlayerAttack. Then I can further down in PlayerAttack compare a specific string with the content in Knowledge<>, and if the string exists there, then do a specific attack.
Any help would be greatly appreciated :)
using UnityEngine;
using System;
using System.Collections;
using System.Collections.Generic;
public class PlayerAttack : MonoBehaviour {
public GameObject target;
private PlayerKnowledge playerKnowledge;
private List<string> knowledge = new List<string>();
// Use this for initialization
void Start () {
//This is the line the error points to
var know = playerKnowledge.knownNames;
//Anyone know how to print content of List<> BTW?
Debug.Log("know returns" + know);
if(know != null){
knowledge.AddRange(know);
Debug.Log("knowledge !null returns: " + knowledge);
}
}
using UnityEngine;
using System.Collections;
using System.Collections.Generic;
public class PlayerKnowledge : MonoBehaviour {
public List<string> knownNames;
public GameObject item;
private GameObject book;
// Use this for initialization
void Start () {
var knownNames = new List<string>();
knownNames.Add("Test");
Debug.Log("PlayerKnowledge Start: " + knownNames);
}
Answer by phodges · Oct 28, 2012 at 06:46 AM
I don't see where playerKnowledge is being initialised in your code. If you don't initialise it then as soon as you try to dereference it to obtain 'knownNames' you will see the exception you describe.
Given the names of your classes, I suspect that they are both found as components on your player object. If that's the case, then use a line like this in PlayerAttack.Start:
playerKnowledge = GetComponent<PlayerKnowledge>();
if (null == playerKnowledge){
Debug.LogError("PlayerKnowledge not found. This code will crash");
}
If you know that an object with a PlayerAttack component must always have a PlayerKnowledge component too, then you might consider using a [RequireComponent(typeof(PlayerKnowledge))] tag to describe your attack component, otherwise add a null test before var know = ...
Glad it helped. Please press the button to accept the answer.
Your answer
Follow this Question
Related Questions
Why do I keep getting NullReferenceException in Unity while iterating over a List (in OnDrawGizmos)? 2 Answers
A node in a childnode? 1 Answer
C# NullReferenceException 2 Answers
My Project doesn't work - Generic.List throws NullReferenceException ?! 0 Answers
How to null-reference check a Collider2D that's been destroyed 1 Answer