What is a Null Reference Exception In Unity.

NullReferenceException are thrown when you try to access a reference variable that isnt referencing any object, hence it is null. So the first question, are all variables reference types?

No, in Mono, there are two kinds of variables that are used for most variables: value types and reference types. Value types store a value directly such as an int or a double or any struct such as a Vector3, or an enumeration.

Reference types, on the other hand to not directly store the object data, instead, they store a reference to an object similar to pointers in C/C++. Common reference types are classes, delegates, and strings. Reference types default to null, that is that they are not referencing any object. Hence, if you try and access the object that is being referenced and their isnt one, you will get a NullReferenceException.

Here is another Answer about null references that explains the same thing in slightly different words.

Some common examples:

 var t : Transform; 
 //t is a reference to a Transform.
 //If you do not assign a Transform to it in the Inspector, then
 //it will be be a null reference..
 
 function Start () {
      t.Translate();
      //If t does not have a Transform, then you will get a NullReferenceException
 }

If you try to get a component that isnt there then try to access it:

 function Start () {
       var c : Light = GetComponent(Light);
 
      c.range = 10;
      //If this object doesnt have a light on it, then you will get
      //a NullReferenceException
 }

Accessing a GameObject that doesnt exist:

 function Start () {
     var someGameObject : GameObject = GameObject.Find("AGameObjectThatDoesntExist");
 
     someGameObject.name = "NullReferenceException";
 }

Less common, but annoying if you don't know it:

 //Note this example is C#. There is no good way of doing delegates in js
 public void NullDelegate (MyEmptyDelegate del) {
      del();
      //if you call a delegate that doesn't have any methods attached, you will get a NullReference Exception.
 }

Fixes

Now to how to fix them: obviously the easiest way to fix them is to not have any NullReferences. But many times thats not possible so you need to have either try-catch blocks or conditionals.

For example:

 var c = GetComponent(MyComponent);
 
 if(c != null) {
     //Make sure we have a reference.
     //Do the rest of the stuff.
 }
 
 else {
     Debug.Log(There is no MyComponent attached to this object);
 }

or:

 try {
      //execute code;
 }
 
 catch (var ex : NullReferenceException) {
       //Do other stuff.
 }

try/catch blocks require that you throw an exception and that does cost resources so more often than not, even though it is sometimes cleaner to read, it isn't as performance efficient.

   if(object.ReferenceEquals(someValue,null))
   {
    //someValue is null
   }