What is a Null Reference Exception?

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 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

Null Checks

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 it null, then we display a message.

Try/Catch Blocks

Another cause for NullReferenceException is to use a variable that should be initialised in the Inspector. 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.

Summary

• NullReferenceException happens when your script code tries to use a variable which isn’t set (referencing) and object.
• The error message that appears tells you a great deal about where in the code the problem happens.
• NullReferenceException can be avoided by writing code that checks for null before accessing an object, or uses try/catch blocks.