A NullReferenceException
happens when you try to access a reference variable that isn’t referencing any object. If a reference variable isn’t referencing an object, then it’ll be treated as null
. The run-time will tell you that you are trying to access an object, when the variable is null
by issuing a NullReferenceException
.
Reference variables in c# and JavaScript are similar in concept to pointers in C and C++. Reference types default to null
to indicate that they are not referencing any object. Hence, if you try and access the object that is being referenced and there isn’t one, you will get a NullReferenceException
.
When you get a NullReferenceException
in your code it means that you have forgotten to set a variable before using it. The error message will look something like:
NullReferenceException: Object reference not set to an instance of an object
at Example.Start () [0x0000b] in /Unity/projects/nre/Assets/Example.cs:10
This error message says that a NullReferenceException
happened on line 10 of the script file Example.cs
. Also, the message says that the exception happened inside the Start()
function. This makes the Null Reference Exception easy to find and fix. In this example, the code is:
//c# example
using UnityEngine;
using System.Collections;
public class Example : MonoBehaviour {
// Use this for initialization
void Start () {
__GameObject__The fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject's functionality is defined by the Components attached to it. [More info](class-GameObject.html)<span class="tooltipGlossaryLink">See in [Glossary](Glossary.html#GameObject)</span> go = GameObject.Find("wibble");
Debug.Log(go.name);
}
}
The code simply looks for a game object called “wibble”. In this example there is no game object with that name, so the Find()
function returns null
. On the next line (line 9) we use the go
variable and try and print out the name of the game object it references. Because we are accessing a game object that doesn’t exist the run-time gives us a NullReferenceException
Although it can be frustrating when this happens it just means the script needs to be more careful. The solution in this simple example is to change the code like this:
using UnityEngine;
using System.Collections;
public class Example : MonoBehaviour {
void Start () {
GameObject go = GameObject.Find("wibble");
if (go) {
Debug.Log(go.name);
} else {
Debug.Log("No game object called wibble found");
}
}
}
Now, before we try and do anything with the go
variable, we check to see that it is not null
. If it is null
, then we display a message.
Another cause for NullReferenceException
is to use a variable that should be initialised in the InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
See in Glossary. If you forget to do this, then the variable will be null
. A different way to deal with NullReferenceException
is to use try/catch block. For example, this code:
using UnityEngine;
using System;
using System.Collections;
public class Example2 : MonoBehaviour {
public Light myLight; // set in the inspector
void Start () {
try {
myLight.color = Color.yellow;
}
catch (NullReferenceException ex) {
Debug.Log("myLight was not set in the inspector");
}
}
}
In this code example, the variable called myLight
is a Light
which should be set in the Inspector window. If this variable is not set, then it will default to null
. Attempting to change the color of the light in the try
block causes a NullReferenceException
which is picked up by the catch
block. The catch
block displays a message which might be more helpful to artists and game designers, and reminds them to set the light in the inspector.
NullReferenceException
happens when your script code tries to use a variable which isn’t set (referencing) and object.NullReferenceException
can be avoided by writing code that checks for null
before accessing an object, or uses try/catch blocks.