What is the meaning of NullReferenceException

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What is a NullReferenceException in .NET?

For example, "System.NullReferenceException was unhandled", with Message "Object reference not set to an instance of an object."

What is the meaning of this exception, and how can it be solved?

score 14 · Answer 1 · edited May 23 '17 at 12:16

14

It means you've tried to access a member of something that isn't there:

string s = null;
int i = s.Length; // boom

Just fix the thing that is null. Either make it non-null, or perform a null-test first.

There is also a corner-case here related to Nullable<T>, generics and the new generic constraint - a bit unlikely though (but heck, I hit this issue!).

edited May 23 '17 at 12:16

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answered Jun 23 '09 at 08:33

Marc Gravell

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score 8 · Answer 2 · answered Jun 23 '09 at 08:33

8

This is the most common exception in .NET... it just mean that you try to call a member of a variable that is not initialized (null). You need to initialize this variable before you can call its members

answered Jun 23 '09 at 08:33

Thomas Levesque

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Note that this advice applies to "fields" (instance/static variables) - but not to local variables (definite assignment is applied to local variables; you can't even *attempt* to use an uninitialized local variable). – Marc Gravell Jun 23 '09 at 08:35

Lloyd · Answer 3 · 2012-12-02T17:43:00.027

2

It means you're referencing something that's null, for example:

class Test
{

   public object SomeProp
   {
      get;
      set;
   }

}

new Test().SomeProp.ToString()

SomeProp will be null and should throw a NullReferenceException. This is commonly due to code you are calling expecting something to be there that isn't.

edited Dec 02 '12 at 17:43

answered Jun 23 '09 at 08:34

Lloyd

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If `o` is a local variable, this won't compile. Unassigned local variables are not `null`. – CodesInChaos Dec 01 '12 at 10:09
You're right. Changed the example. – Lloyd Dec 02 '12 at 17:43

score 1 · Answer 4 · answered Jun 23 '09 at 08:33

1

That means that you have tried to use a method or property of an object, when the variable is not yet initialized:

string temp;
int len = temp.Length; // throws NullReferenceException; temp is null

string temp2 = "some string";
int len2 = temp2.Length; // this works well; temp is a string

answered Jun 23 '09 at 08:33

Fredrik Mörk

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For local variables (as per the example shown), it *is* initialized (otherwise it won't compile). Simply: it is initialized to null. – Marc Gravell Jun 23 '09 at 08:39
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if 'temp' is a local variable it will _not_ compile. If it's a field it will be null. – H H Jun 23 '09 at 08:47

score 1 · Answer 5 · answered Jun 23 '09 at 08:34

1

The code below will show you the exception, and a clue.

string s = null;
s = s.ToUpper();

answered Jun 23 '09 at 08:34

H H

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Matt Jacobsen · Answer 6 · 2009-06-23T08:50:05.353

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Somewhere in your code, you have an object reference, and it's not set to an instance of an object :)

Somewhere you've used an object without calling it's constructor.

what you should do:

MyClass c = new MyClass();

what you've done:

MyClass c;
c.Blah();

edited Jun 23 '09 at 08:50

answered Jun 23 '09 at 08:34

Matt Jacobsen

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Which won't compile... (definite assignment) – Marc Gravell Jun 23 '09 at 08:36
You're right, and they won't have my implementation of MyClass either. Let's do Random r = null; r.Next(); instead then. – Matt Jacobsen Jun 23 '09 at 08:49

What is the meaning of NullReferenceException

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